Calling mdiobus_detect() for a bus that serves as a parent for a MDIO
bus multiplexer would result in parent bus being populated with
devices that are present on downstream bus that multiplexer happens to
be "pointing" at that moment. To avoid that introduce 'is_multiplexed'
flag to struct mii_bus and change mdiobus_detect to ignore busses for
which that flag is set.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>