PCI: xilinx: Fix NULL pointer dereference in xilinx_pcie_intr_handler()

f29861aa ("PCI: xilinx: Switch to msi_create_parent_irq_domain()")
changed xilinx_pcie::msi_domain from child devices' interrupt domain to
Xilinx AXI bridge's interrupt domain.

However, xilinx_pcie_intr_handler() wasn't changed and still reads Xilinx
AXI bridge's interrupt domain from xilinx_pcie::msi_domain->parent. This
pointer is NULL now.

Update xilinx_pcie_intr_handler() to read the correct interrupt domain
pointer.

Fixes: f29861aa ("PCI: xilinx: Switch to msi_create_parent_irq_domain()")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250811054144.4049448-1-namcao@linutronix.de