net: stmmac: use .mac_select_pcs() interface

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

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