net: ethernet: Add helper for MACs which support asym pause

Rather than have the MAC drivers manipulate phydev members to indicate
they support Asym Pause, add a helper function.

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>