wl12xx: rename driver to wl1251

Rename driver to wl1251.ko in preparation for wl1271 driver.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>