net: stmmac: stm32: convert to use phy_interface

net: stmmac: stm32: convert to use phy_interface

dwmac-stm32 supports MII, RMII, GMII and RGMII interface modes,
selecting the dwmac core interface mode via bits 23:21 of the
SYSCFG register. The bit combinations are identical to the
dwmac phy_intf_sel_i signals.

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

Convert dwmac-stm32 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/E1uytpf-00000006H2c-3hiU@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>