usb: core: rename mutex usb_bus_list_lock to usb_bus_idr_lock

Now that usb_bus_list has been removed and switched to idr
rename the related mutex accordingly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>