rt2x00: rt2800pci: use rt2800mmio prefix for initialization functions

The functions are used for devices with memory
mapped I/O and contain no PCI specific code at
all. Use rt2800mmio prefix instead of rt2800pci
in the function names to reflect that.

The patch contains no functional changes.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>