net: sparx5: use .mac_select_pcs() interface

Convert sparx5 to use the mac_select_interface rather than using
phylink_set_pcs(). The intention here is to unify the approach for
PCS and eventually remove phylink_set_pcs().

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