net: phylink: remove use of pl->pcs in phylink_validate_mac_and_pcs()

When the mac_select_pcs() method is not implemented, there is no way
for pl->pcs to be set to a non-NULL value. This was here to support
the old phylink_set_pcs() method which has been removed a few years
ago. Simplify the code in phylink_validate_mac_and_pcs().

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