of: of_mdio: Ensure mdio device is a PHY

of_phy_find_device() is used to find the phy device associated with a
device node. It is expected the node is for a PHY device, but in fact
it could of been probed as a generic MDIO device. Ensure the device is
a PHY before returning it.

Fixes: a9049e0c ("mdio: Add support for mdio drivers.")
Reported-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>