wl12xx: check if elp wakeup failed

Check the return call from wl12xx_ps_elp_wakeup() and bail out if it
fails. This shouldn't happen, but if does there's a fundamental low
level issue.

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>