The root bus checks rework in d84c572d ("PCI: rockchip: Use
pci_is_root_bus() to check if bus is root bus") caused a regression whereby
in rockchip_pcie_valid_device() if the bus parameter is the root bus and
the dev value == 0, the function should return 1 (ie true) without checking
if the bus->parent pointer is a root bus because that triggers a NULL
pointer dereference.

Fix this by streamlining the root bus detection.

Fixes: d84c572d ("PCI: rockchip: Use pci_is_root_bus() to check if bus is root bus")
Link: https://lore.kernel.org/r/20200904140904.944-1-lorenzo.pieralisi@arm.comReported-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Tested-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Shawn Lin <shawn.lin@rock-chips.com>