PCI: rockchip: Fix bus checks in rockchip_pcie_valid_device()

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>

drivers/pci/controller/pcie-rockchip-host.c

@@ -71,16 +71,13 @@ static void rockchip_pcie_update_txcredit_mui(struct rockchip_pcie *rockchip)
 static int rockchip_pcie_valid_device(struct rockchip_pcie *rockchip,
 				      struct pci_bus *bus, int dev)
 {
-	/* access only one slot on each root port */
-	if (pci_is_root_bus(bus) && dev > 0)
-		return 0;
-
 	/*
-	 * do not read more than one device on the bus directly attached
+	 * Access only one slot on each root port.
+	 * Do not read more than one device on the bus directly attached
 	 * to RC's downstream side.
 	 */
-	if (pci_is_root_bus(bus->parent) && dev > 0)
-		return 0;
+	if (pci_is_root_bus(bus) || pci_is_root_bus(bus->parent))
+		return dev == 0;
 
 	return 1;
 }