ieee802154: Fix oops during ieee802154_sock_ioctl

Trying to run izlisten (from lowpan-tools tests) on a device that does not
exists I got the oops below. The problem is that we are using get_dev_by_name
without checking if we really get a device back. We don't in this case and
writing to dev->type generates this oops.

[Oops code removed by Dmitry Eremin-Solenikov]

If possible this patch should be applied to the current -rc fixes branch.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>