pci: pci_mvebu: Fix write_config() with PCI_SIZE_8 or PCI_SIZE_16

Current implementation of write_config() is broken for PCI_SIZE_8 or
PCI_SIZE_16 as it always uses writel(), which means that write operation
is always 32-bit, so upper 24 bits for PCI_SIZE_8 and upper 16 bits for
PCI_SIZE_16 are cleared.

Fix this by using writeb() and writew(), respectively.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>