PCI: brcmstb: Assign pcie->gen from of_pci_get_max_link_speed()

PCI: brcmstb: Assign pcie->gen from of_pci_get_max_link_speed()

After commit 03f920936977 ("PCI: controller: Validate max-link-speed"),
pcie->gen stopped being assigned and as a result the established PCIe link
would stop supporting Gen3 speeds on 2712 since pcie->gen is used to
populate LnkCntl2 and LnkCap in brcm_pcie_set_gen().

If the 'max-link-speed' property is not specified, or it exceeds Gen3,
resort to the HW defaults.

Link: https://github.com/raspberrypi/linux/issues/7343
Reported-by: Dom Cobley <popcornmix@gmail.com>
Reported-by: Phil Elwell <phil@raspberrypi.com>
Fixes: 03f920936977 ("PCI: controller: Validate max-link-speed")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Hans Zhang <18255117159@163.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20260506164537.103196-1-florian.fainelli@broadcom.com