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.

Also, bridge->busnr is never set so effectively the root bus must be 0.
This will be fixed by a subsequent commit.

Link: https://lore.kernel.org/r/20200722022514.1283916-10-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Shawn Lin <shawn.lin@rock-chips.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: linux-rockchip@lists.infradead.org