ieee802154: fix error handling in ieee802154fake_probe()

In case of any failure ieee802154fake_probe() just calls unregister_netdev().
But it does not look safe to unregister netdevice before it was registered.

The patch implements straightforward resource deallocation in case of
failure in ieee802154fake_probe().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>