isdn: remove duplicate NULL check

We test both "!skb_out" and "skb_out" here which is duplicative and
causes a static checker warning.  I considered that the intent might
have been to test "skb_in" but that's a valid pointer here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>