drivers: net: xgene: Workaround for HW errata 10GE_4

This patch adds workaround for HW errata 10GE_4:
"XGENET_ICM_ECM_DROP_COUNT_REG_0 reg not clear on read".

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>