net: stmmac/xpcs: convert to pcs_validate()

stmmac explicitly calls the xpcs driver to validate the ethtool
linkmodes. This is no longer necessary as phylink now supports
validation through a PCS method. Convert both drivers to use this
new mechanism.

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