net: stmmac: imx: convert to use phy_interface

net: stmmac: imx: convert to use phy_interface

Checking the IMX8MP documentation, there is no requirement for a
separate mac_interface mode definition. As mac_interface and
phy_interface will be the same, use phy_interface internally 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/E1uytpL-00000006H2F-1o6b@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>