ramips: fix gigabit switch PHY access on MDIO

When PHY's are defined on the MDIO bus in the DTS, gigabit support was
being masked out for no apparent reason, pegging all such ports to 10/100.
If gigabit support must be disabled for some reason, there should be a
"max-speed" property in the DTS.

Reported-by: James McKenzie <openwrt@madingley.org>
Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>

target/linux/ramips/files-4.14/drivers/net/ethernet/mediatek/mdio.c

@@ -110,7 +110,8 @@ static void phy_init(struct fe_priv *priv, struct phy_device *phy)
 	phy->autoneg = AUTONEG_ENABLE;
 	phy->speed = 0;
 	phy->duplex = 0;
-	phy->supported &= PHY_BASIC_FEATURES;
+	phy->supported &= IS_ENABLED(CONFIG_NET_MEDIATEK_MDIO_MT7620) ?
+			PHY_GBIT_FEATURES : PHY_BASIC_FEATURES;
 	phy->advertising = phy->supported | ADVERTISED_Autoneg;
 
 	phy_start_aneg(phy);