net: phy: add phy_modify_paged_changed

Add helper function phy_modify_paged_changed, behavios is the same
as for phy_modify_changed.

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