USB: make vendor id of root hubs greppable

It took me surprisingly long to find the location where the Linux
Foundation vendor id (0x1d6b) is set for the root hubs. A minor update
to three comments makes those locations (trivially) greppable.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>