drivers: net: xgene: fix RGMII 10/100Mb mode

This patch fixes the RGMII 10/100M mode by reprogramming the clock.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>