wl1251: separate bus i/o code into io.c

In order to eventually support wl1251 spi and sdio interfaces, move
the register and memory transfer functions to a common file.  Also
rename wl1251_spi_mem_{read,write} to indicate its common usage.
We still use spi_read internally until SDIO interface is introduced
so nothing functional should change here.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>