wl1251: rename reg.h to wl1251_reg.h

Now that wl1271 doesn't use reg.h anymore, it can be renamed to
wl1251_reg.h.

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