net: phylink: provide phylink_validate_mask_caps() helper

Provide a helper that restricts the link modes according to the
phylink capabilities.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
[rebased on net-next/master and added documentation]
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>