wl12xx: Make sure HW is available in sched scan ops

The sched_scan_(stop|start) ops fails to check for WL1271_STATE_OFF.
This can lead to a race where the driver tries to access the HW
while it's off.

Fix this by checking for WL1271_STATE_OFF before accessing the HW.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>