net: stmmac: sun8i: convert to use phy_interface

net: stmmac: sun8i: convert to use phy_interface

dwmac-sun8i supports MII, RMII and RGMII interface modes only. It
is unclear whether the dwmac core interface is different from the
one presented to the outside world.

However, as none of the DTS files set "mac-mode", mac_interface will
be identical to phy_interface.

Convert dwmac-sun8i to use phy_interface when determining the
interface mode rather than mac_interface.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://patch.msgid.link/E1uytpl-00000006H2k-08pH@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>