PCI: loongson: Do not ignore downstream devices on external bridges

PCI: loongson: Do not ignore downstream devices on external bridges

Loongson PCI host controllers have a hardware quirk that requires
software to ignore downstream devices with device number > 0 on the
internal bridges. The current implementation applies the workaround to
all non-root buses, which breaks external bridges (e.g., PCIe switches)
with multiple downstream devices.

Fix it by only applying the workaround to internal bridges.

Tested on Loongson-LS3A4000-7A1000-NUC-SE, using AMD Promontory 21
chipset add-in card [1].

  $ lspci -tnnnvvv
  -[0000:00]-+-00.0  Loongson Technology LLC 7A1000 Chipset Hyper Transport Bridge Controller [0014:7a00]
             +-00.1  Loongson Technology LLC 7A2000 Chipset Hyper Transport Bridge Controller [0014:7a10]
             +-03.0  Loongson Technology LLC 2K1000/2000 / 7A1000 Chipset Gigabit Ethernet Controller [0014:7a03]
             +-04.0  Loongson Technology LLC 2K1000 / 7A1000/2000 Chipset USB OHCI Controller [0014:7a24]
             +-04.1  Loongson Technology LLC 2K1000 / 7A1000/2000 Chipset USB EHCI Controller [0014:7a14]
             +-05.0  Loongson Technology LLC 2K1000 / 7A1000/2000 Chipset USB OHCI Controller [0014:7a24]
             +-05.1  Loongson Technology LLC 2K1000 / 7A1000/2000 Chipset USB EHCI Controller [0014:7a14]
             +-06.0  Loongson Technology LLC 7A1000 Chipset Vivante GC1000 GPU [0014:7a15]
             +-06.1  Loongson Technology LLC 2K1000 / 7A1000 Chipset Display Controller [0014:7a06]
             +-07.0  Loongson Technology LLC 2K1000/2000/3000 / 3B6000M / 7A1000/2000 Chipset HD Audio Controller [0014:7a07]
             +-08.0  Loongson Technology LLC 2K1000 / 7A1000 Chipset 3Gb/s SATA AHCI Controller [0014:7a08]
             +-08.1  Loongson Technology LLC 2K1000 / 7A1000 Chipset 3Gb/s SATA AHCI Controller [0014:7a08]
             +-08.2  Loongson Technology LLC 2K1000 / 7A1000 Chipset 3Gb/s SATA AHCI Controller [0014:7a08]
             +-09.0-[01]----00.0  Qualcomm Technologies, Inc QCNFA765 Wireless Network Adapter [17cb:1103]
             +-0a.0-[02]----00.0  Etron Technology, Inc. EJ188/EJ198 USB 3.0 Host Controller [1b6f:7052]
             +-0f.0-[03-08]----00.0-[04-08]--+-00.0-[05]----00.0  Shenzhen Longsys Electronics Co., Ltd. FORESEE XP1000 / Lexar Professional CFexpress Type B Gold series, NM620 PCIe NVME SSD (DRAM-less) [1d97:5216]
             |                               +-08.0-[06]----00.0  MAXIO Technology (Hangzhou) Ltd. NVMe SSD Controller MAP1202 (DRAM-less) [1e4b:1202]
             |                               +-0c.0-[07]----00.0  Advanced Micro Devices, Inc. [AMD] 600 Series Chipset USB 3.2 Controller [1022:43f7]
             |                               \-0d.0-[08]----00.0  Advanced Micro Devices, Inc. [AMD] 600 Series Chipset SATA Controller [1022:43f6]
             \-16.0  Loongson Technology LLC 7A1000 Chipset SPI Controller [0014:7a0b]

Fixes: 2410e3301fcc ("PCI: loongson: Don't access non-existent devices")
Co-developed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: Lain "Fearyncess" Yang <i@lain.vg>
Signed-off-by: Lain "Fearyncess" Yang <i@lain.vg>
Signed-off-by: Rong Zhang <i@rong.moe>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://oshwhub.com/wesd/b650
Link: https://patch.msgid.link/20260501-ls7a-bridge-fixes-v2-1-69fa93683805@rong.moe