Rather than have MAC drivers open code the test, add a helper in
phylib. This will help when we change the type of phydev->supported.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>