net: dsa: mt7530: drop use of phylink_helper_basex_speed()

Now that we have a better method to select SFP interface modes, we
no longer need to use phylink_helper_basex_speed() in a driver's
validation function.

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>