staging: comedi vmk80xx: lock held on error path

If the user passes an invalid command, then we don't drop the lock
before returning.  The check for invalid commands doesn't need to be
done under lock so I moved it forward a couple lines.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>