arm: a37xx: pci: Ensure that root port is always on root zero bus

Writing to the PCI_PRIMARY_BUS register of the root port should not change
bus number on which is root port present.

This PCI_PRIMARY_BUS register is used only for correct configuration of
legacy PCI stuff, like forwarding of PCI special cycles between buses.

Aardvark HW does not support PCI special cycles, so it does not have HW
register for PCI_PRIMARY_BUS and therefore it does not matter what is
stored in this register.

So fix this issue and do not use PCI_PRIMARY_BUS register in pci-aardvark.c
driver for moving root bus of the root port.

After this change there is no reason for storing bus number (zero) into
first_busno variable, so remove this variable.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: cb056005 ("arm: a37xx: pci: Add support for accessing PCI Bridge on root bus")
Reviewed-by: Stefan Roese <sr@denx.de>