drivers: net: xgene: Remove redundant local stats

Commit 5944701d ("net: remove useless memset's in drivers get_stats64")
makes the pdata->stats redundant. This patch removes pdata->stats and
updates get_stats64() callback accordingly.

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>