net: mtk_eth_soc: move trgmii ddr2 check to probe function

If TRGMII mode is not permitted when using DDR2 mode, we should handle
that when setting up phylink's ->supported_interfaces so phylink knows
that this is not supported by the hardware. Move this check to
mtk_add_mac().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>