of_mdio: use PTR_ERR_OR_ZERO()

PTR_ERR_OR_ZERO() is open coded in of_phy_register_fixed_link(), so just
call it directly...

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