rt2x00: rt2500pci: use the rt2x00mmio_* routines

Use the recently introduced rt2x00mmio_* routines
instead of the rt2x00pci_* variants.

The patch contains no functional changes.
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>