rionet: fix NULL pointer dereference in rionet_remove

Function rionet_remove initializes local variable 'ndev' to NULL
and do nothing changes before the call to unregister_netdev(ndev),
this could cause a NULL pointer dereference.

Reported-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Yinglin Luan <synmyth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>