wl1251: move module probe methods into spi.c

This change moves all of the spi specific code from main.c into
spi.c.  The module initialization code also moves, but common
code for initializing mac80211 etc. stays in main.c, as this will
eventually form a common library module also used by wl1251_sdio.

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>