net: mtk_eth_soc: 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, and we can also get rid of our hack to indicate
both 1000base-X and 2500base-X if the comphy is present to make that
work. Remove this hack and use of phylink_helper_basex_speed().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>