net: macb: use .mac_select_pcs() interface

Convert the PCS selection to use mac_select_pcs, which allows the PCS
to perform any validation it needs.

We must use separate phylink_pcs instances for the USX and SGMII PCS,
rather than just changing the "ops" pointer before re-setting it to
phylink as this interface queries the PCS, rather than requesting it
to be changed.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>