net: dsa: remove dsa_port_phylink_mac_select_pcs()

There is no longer any reason to implement the mac_select_pcs()
callback in DSA. Returning ERR_PTR(-EOPNOTSUPP) is functionally
equivalent to not providing the function.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>