drivers: net: xgene: Add rx_overrun/tx_underrun statistics

This patch adds rx_overrun and tx_underrun ethtool statistic counters.

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