[media] usbvision-video: two use after frees

commit 470a9147 upstream.

The lock has been freed in usbvision_release() so there is no need to
call mutex_unlock() here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>