net: stmmac: remove mac_interface

net: stmmac: remove mac_interface

mac_interface has served little purpose, and has only caused confusion.
Now that we have cleaned up all platform glue drivers which should not
have been using mac_interface, there are no users remaining. Remove
mac_interface.

This results in the special dwmac specific "mac-mode" DT property
becoming redundant, and an in case, no DTS files in the kernel make use
of this property. Add a warning if the property is set, and it is
different from the "phy-mode".

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Link: https://patch.msgid.link/E1uytpv-00000006H2x-196h@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>