of_mdio: use IS_ERR_OR_NULL()

IS_ERR_OR_NULL() is open coded in of_mdiobus_register_phy(), so just call
it directly...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>