net: stmmac: convert to phylink_generic_validate()

Convert stmmac to use phylink_generic_validate() now that we have the
MAC capabilities and supported interfaces filled in, and we have the
PCS validation handled via the PCS operations.

Tested-by: Wong Vee Khee &lt;vee.khee.wong@linux.intel.com&gt; # Intel EHL Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>