6lowpan: release device on error path

We recently added a new error path and it needs a dev_put().

Fixes: 7adac1ec ('6lowpan: Only make 6lowpan links to IEEE802154 devices')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>