x86/PCI: Check pcie_find_root_port() return for NULL

x86/PCI: Check pcie_find_root_port() return for NULL

If pcie_find_root_port() is unable to locate a Root Port, it will return
NULL. Check the pointer for NULL before dereferencing it.

This particular case is in a quirk for devices that are always below a Root
Port, so this won't avoid a problem and doesn't need to be backported, but
check as a matter of style and to prevent copy/paste mistakes.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Samasth Norway Ananda <[email protected]>
[bhelgaas: drop Fixes: and explain why there's no problem in this case]
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>