net: phylink: update to use phy_support_asym_pause()

Use phy_support_asym_pause() rather than open-coding it.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>