PCI: stm32: Fix LTSSM EP race with start link

PCI: stm32: Fix LTSSM EP race with start link

If the host has deasserted PERST# and started link training before the link
is started on EP side, enabling LTSSM before the endpoint registers are
initialized in the perst_irq handler results in probing incorrect values.

Thus, wait for the PERST# level-triggered interrupt to start link training
at the end of initialization and cleanup the stm32_pcie_[start stop]_link
functions.

Fixes: 151f3d29baf4 ("PCI: stm32-ep: Add PCIe Endpoint support for STM32MP25")
Signed-off-by: Christian Bruel <christian.bruel@foss.st.com>
[mani: added fixes tag]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: wrap line]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251114-perst_ep-v1-1-e7976317a890@foss.st.com