bcma: don't map/unmap a subset of the PCI config space

For PCI config space access offsets < 256 for device '0',
bcma_extpci_write_config performs an 'ioremap_nocache' on a 4 byte
section of the PCI config space (an area that has already
previously been mapped), and then subsequently unmaps that 4 byte
section.  This can't be a good thing for future read access from
that now unmapped location.  Modify the config space writes to use
the existing access functions (similar to how it is done for the reads).

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>