USB: correct the usb_disconnect() comment about usb_bus_list_lock

usb_disconnect() no longer acquires usb_bus_list_lock, so update its
comment to that effect.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>