of_mdio: make of_mdiobus_register_{device|phy}() *void*

The results of of_mdiobus_register_{device|phy}() are never checked, so we
can make  both these functions *void*...

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