net: dsa: b53: switch to using phylink_generic_validate()

Switch the Broadcom b53 driver to using the phylink_generic_validate()
implementation by removing its own .phylink_validate method and
associated code.

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