net: dsa: sja1105: convert to phylink_generic_validate()

Populate the MAC capabilities for the SJA1105 DSA switch using the same
decision making which sja1105_phylink_validate() uses. Remove the now
obsolete sja1105_phylink_validate() implementation to allow DSA to use
phylink_generic_validate() for this switch driver.

As noted by Vladimir, this fixes an inconsequential bug which allowed
gigabit and lower interface modes to be indicated when operating in
2500base-X mode.

Reviewed-by: Vladimir Oltean &lt;olteanv@gmail.com&gt; Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>