wl1251: make wl1251_set_partition bus agnostic

The same partition setting code can be used for both SPI and SDIO
modes, if we remove the spi-specific commands and use the more
generic buffer write routines.  Do that and move it to io.c
since it deals with register/memory address offsets.

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>