A NULL pointer dereference is possible for the argument ring->buf_pool
which is passed to xgene_enet_free_desc_ring(), as ring could be NULL.

And now since NULL pointers are being checked for before the calls to
xgene_enet_free_desc_ring(), might as well take advantage of them and
not call the function if the argument would be NULL.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NIyappan Subramanian <isubramanian@apm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>