mdio_bus: Add comment to mdiobus_scan() and __mdiobus_register()

Make it clear that mdiobus_scan () will only find devices which have a
vendor/product ID in registers 2 and 3. These are typically PHY
devices. Other sort of MDIO devices, such as switches, are not
expected to be found during the scan.

Similarly, __mdiobus_register(), which calls mdiobus_scan() will only
find PHY devices, and other sorts of MDIO devices are expected to be
instantiated from device tree.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>