net: dsa: ocelot: convert to mac_select_pcs()

Convert the PCS selection to use mac_select_pcs, which allows the PCS
to perform any validation it needs, and removes the need to set the PCS
in the mac_config() callback, delving into the higher DSA levels to do
so.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>