net: stmmac: thead: convert to use phy_interface

net: stmmac: thead: convert to use phy_interface

dwmac-thead supports either MII or RGMII interface modes only.

None of the DTS files set "mac-mode", so mac_interface will be
identical to phy_interface.

Convert dwmac-thead to use phy_interface when determining the
interface mode rather than mac_interface.

Also convert the error prints to use phy_modes() so that we get a
meaningful string rather than a number for the interface mode.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uytpq-00000006H2q-0ajY@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>