carl9170: Fix mismatch in carl9170_op_set_key mutex lock-unlock

If is_main_vif(ar, vif) reports that we have to fall back
to software encryption, we goto err_softw; before locking ar->mutex.
As a result, we have unprotected call to carl9170_set_operating_mode
and unmatched mutex_unlock.

The patch fix the issue by adding mutex_lock before goto.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: <stable@kernel.org>
Acked-By: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>