net: dsa: b53: 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.

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