rtnetlink: remove an unneeded test

We know that "dev" is a valid pointer at this point, so we can remove
the test and clean up a little.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>