econet: Do the correct cleanup after an unprivileged SIOCSIFADDR.

We need to drop the mutex and do a dev_put, so set an error code and break like
the other paths, instead of returning directly.

Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: David S. Miller <davem@davemloft.net>