wl1251: rename spi device to wl1251

During rename of the driver from wl12xx to wl1251 the spi device name
was accidentally left as wl12xx. Rename it to wl1251 which is the proper
name of the driver.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>