net: phy: add helper phy_polling_mode

Add a helper for checking whether polling is used to detect PHY status
changes.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>