xgene: Check all RGMII phy mode variants

This patch addresses the review comment from the previous patch set,
by using phy_interface_mode_is_rgmii() helper function to address
all RGMII phy mode variants.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>