Documentation sysfs-bus-usb: Correct use of devnum

Correct use of devnum in supports_autosuspend documentation, the sysfs path
contains busnum-port.port.port not busnum-devnum (which is the usb bus device
address).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>