net: phy: avoid unneeded MDIO reads in genphy_read_status

Considering that in polling mode each link drop will be latched,
settings can't have changed if link was up and is up.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>