net: stmmac: mediatek: use stmmac_get_phy_intf_sel()

net: stmmac: mediatek: use stmmac_get_phy_intf_sel()

Use stmmac_get_phy_intf_sel() to decode the PHY interface mode to the
phy_intf_sel value, validate the result, and pass that into the
implementation specific ->dwmac_set_phy_interface() method. Use this
to configure the PHY interface selection field.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vIjTz-0000000DqtP-3N9v@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>