wl1251: split spi interface into separate module

This creates a module called wl1251_spi.ko which contains just the
SPI-specific code.  The core remains in the module wl1251.ko.

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>