macb: fix mdiobus_scan() error check

Now mdiobus_scan() returns ERR_PTR(-ENODEV) instead of NULL if the PHY
device ID was read as all ones. As this was not  an error before, this
value  should be filtered out now in this driver.

Fixes: b74766a0 ("phylib: don't return NULL from get_phy_device()")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>