docbook: fix usb content

Fix USB docbook warnings.

Warning(linux-2.6.23-git8//include/linux/usb/gadget.h:487): No description found for parameter 'g'
Warning(linux-2.6.23-git8//include/linux/usb/gadget.h:506): No description found for parameter 'g'

Warning(linux-2.6.23-git8//drivers/usb/core/hub.c:1416): No description found for parameter 'usb_dev'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>