wl1251: introduce wl1251_if_operations struct

Introduce an ops struct with read, write, and reset functions to
abstract away the details of the wl1251 bus interface.  Doing this
will allow SDIO to coexist with SPI by supplying its own I/O
routines.

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>