[media] drxk: Does not unlock mutex if sanity check failed in scu_command()

If sanity check fails in scu_command(), goto error leads to unlock of
an unheld mutex. The check should not fail in reality, but it nevertheless
worth fixing.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>