misdn: Fix reversed 'if' in st_own_ctrl

The current code probably returns -EINVAL a lot.  Otherwise it would oops.

Compile tested only.  Found by smatch (http://repo.or.cz/w/smatch.git).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>