wireless: wl12xx, fix lock imbalance

Add omitted mutex_unlock to one of wl12xx_op_start fail paths (when
wl12xx_chip_wakeup fails).

[v2]
Power off the device, because:
\= cite from http://marc.info/?l=linux-kernel&m=124755028209880&w=2
If the chip cannot be booted, why should it remain powered on?
In some rare cases, the chip might fail to initialize, but can
recover if powered off and on again, so turning it off at this
point is the right thing to do. =/

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>