net: mtk_eth_soc: remove unnecessary checks in mtk_mac_config()

mtk_mac_config() checks that the interface mode is permitted for the
capabilities, but we already do these checks in mtk_add_mac() when
initialising phylink's supported_interfaces bitmap. Remove the
unnecessary tests.

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