net: dsa: mt7530: remove interface checks

As phylink checks the interface mode against the supported_interfaces
bitmap, we no longer need to validate the interface mode, nor handle
PHY_INTERFACE_MODE_NA in the validation function. Remove these to
simplify the implementation.

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