pci: mvebu: Ensure that root port is always on root zero bus

pci: mvebu: 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.

Same change and exactly same fix as was done in commit for pci-aardvark.c.

Fixes: 18e57fe33755 ("pci: pci_mvebu: Properly configure and use PCI Bridge (PCIe Root Port)")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>