net: ethernet: mediatek: Fix MT7629 missing GMII mode support

In the original design, mtk_phy_connect function will set ge_mode=1
if phy-mode is GMII(PHY_INTERFACE_MODE_GMII) and then set the correct
ge_mode to ETHSYS_SYSCFG0 register. This logic was broken after apply
mediatek PHYLINK patch(Fixes tag), the new mtk_mac_config function will
not set ge_mode=1 for GMII mode hence the final ETHSYS_SYSCFG0 setting
will be incorrect for mt7629 GMII mode. This patch add the missing logic
back to fix it.

Fixes: b8fc9f30

 ("net: ethernet: mediatek: Add basic PHYLINK support")
Signed-off-by: MarkLee <Mark-MC.Lee@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>