drivers: net: xgene: Fix error deconstruction path

Since register_netdev() call in xgene_enet_probe() was moved down to
the end, it doesn't properly handle errors that may occur, by
deconstructing everything that was setup before the error occurred.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>