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

When reading 8 or 16 bits from config space, use appropriate readb() or
readw() calls. This ensures that PCIe controller does not read more bits
from endpoint card as asked by read_config() function.

Technically there should not be an issue with reading data from config
space which are not later used as there are no clear-by-read registers.
But it is better to use correct read operation based on requested size.

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