of: mdio: Fall back to mdiobus_register() with NULL device_node

When the device_node specified is NULL, fall back to mdiobus_register().
We have a number of drivers having a similar pattern which is:

if (np)
	of_mdiobus_register()
else
	mdiobus_register()

so incorporate that behavior within the core of_mdiobus_register()
function. This is also consistent with the stub version that we defined
when CONFIG_OF=n.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>