drivers: net: xgene: fix register offset

This patch fixes SG_RX_DV_GATE_REG_0_ADDR register offset
and ring state field lengths.

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