net: stmmac: starfive: convert to use phy_interface

net: stmmac: starfive: convert to use phy_interface

dwmac-starfive uses RMII or RGMII interface modes without any PCS,
and selects the dwmac core accordingly using a register field with
the same bit encoding as the core's phy_intf_sel_i signals.

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

Convert dwmac-starfive 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/E1uytpa-00000006H2X-3GWx@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>