mdiobus_register will search for any mdiobus board info registered for
the bus being registered. If found, it will probe devices on the bus.
That device, if for example it is an ethernet switch, may then try to
register an mdio bus. Thus we need to allow recursive calls to
mdiobus_register.

Holding the mdio_board_lock will cause a deadlock during this
recursion. Release the lock and use list_for_each_entry_safe.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>