net: phylink: add pcs_validate() method

Add a hook for PCS to validate the link parameters. This avoids MAC
drivers having to have knowledge of their PCS in their validate()
method, thereby allowing several MAC drivers to be simplfied.

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