drivers: net: xgene: fix statistics counters race condition

This patch fixes the race condition on updating the statistics
counters by moving the counters to the ring structure.

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