X25: Fix oops and refcnt problems from x25_dev_get

Calls to x25_dev_get check for dev = NULL which was not set.
It allowed x25 to set routes and ioctls on down interfaces.
This caused oopses and refcnt problems on device_unregister.

Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>