ath79: support "rgmii-id" PHY mode in ag71xx

The ag71xx code did not include a case for the "rgmii-id" PHY mode in the
code. There are devices that need this mode, so I'm adding it.

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>

target/linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -581,6 +581,7 @@ static void ath79_mii0_ctrl_set_if(struct ag71xx *ag)
 		mii_if = AR71XX_MII0_CTRL_IF_GMII;
 		break;
 	case PHY_INTERFACE_MODE_RGMII:
+	case PHY_INTERFACE_MODE_RGMII_ID:
 		mii_if = AR71XX_MII0_CTRL_IF_RGMII;
 		break;
 	case PHY_INTERFACE_MODE_RMII:
@@ -603,6 +604,7 @@ static void ath79_mii1_ctrl_set_if(struct ag71xx *ag)
 		mii_if = AR71XX_MII1_CTRL_IF_RMII;
 		break;
 	case PHY_INTERFACE_MODE_RGMII:
+	case PHY_INTERFACE_MODE_RGMII_ID:
 		mii_if = AR71XX_MII1_CTRL_IF_RGMII;
 		break;
 	default: