wl12xx: enable ELP mode

ELP mode is working now, let's enable it so that the firmware can sleep
and reduce power consumption.

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