net: ena: remove set but not used variable 'rx_ring'

drivers/net/ethernet/amazon/ena/ena_netdev.c: In function ena_xdp_xmit_buff:
drivers/net/ethernet/amazon/ena/ena_netdev.c:316:19: warning:
 variable rx_ring set but not used [-Wunused-but-set-variable]

commit 548c4940 ("net: ena: Implement XDP_TX action")
left behind this unused variable.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>