wl1271: Moved common IO functions from wl271_spi.c to wl1271_io.c

In prepraration for integration of SDIO implementation moved some IO
functions common for SPI and SDIO to separate file.

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>