mdio_bus: don't return NULL from mdiobus_scan()

mdio_bus: don't return NULL from mdiobus_scan()

I've finally noticed that mdiobus_scan() also returns either NULL or error
value on failure.  Return ERR_PTR(-ENODEV) instead of NULL since this is
the  error value  already filtered out by the callers that want to ignore
the  MDIO address scan failure...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>