net: stmmac: socfpga: convert to use phy_interface

net: stmmac: socfpga: convert to use phy_interface

dwmac-socfpga uses MII, RMII, GMII, RGMII, SGMII and 1000BASE-X
interface modes, and supports the Lynx PCS. The Lynx PCS will only be
used for SGMII and 1000BASE-X modes, with the MAC programmed to use
GMII or MII mode to talk to the PCS. This suggests that the Synopsys
optional dwmac PCS is not present.

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

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

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uytpV-00000006H2R-2nA6@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>