net: greth: use phy_read_status()

In case the greth driver is bound to anything but the Generic PHY
driver or the PHY has a special read_status callback implemented,
unexpected things will happen. Make sure we that we use
phy_read_status() which does the proper abstraction of calling the
driver specific read_status() callback for a given PHY.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>