PCI: xilinx: Use pci_is_root_bus() to check if bus is root bus

PCI: xilinx: Use pci_is_root_bus() to check if bus is root bus

Use pci_is_root_bus() rather than tracking the root bus number to
determine if the bus is the root bus or not. This removes storing
duplicated data as well as the need for the host bridge driver to have
to care about the bus numbers in most cases.

There was also a bug that the root_busno is never set which means the
root bus number is always 0 even if the DT said something else.

Link: https://lore.kernel.org/r/20200722022514.1283916-9-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: linux-pci@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org