usb: gadget: Add kerneldoc for some neglected structure fields

A couple of structures in <linux/usb/gadget.h> have incomplete kerneldoc
comments, leading to these warnings in the docs build:

./include/linux/usb/gadget.h:230: warning: No description found for parameter 'claimed'
./include/linux/usb/gadget.h:230: warning: No description found for parameter 'enabled'
./include/linux/usb/gadget.h:412: warning: No description found for parameter 'quirk_altset_not_supp'
./include/linux/usb/gadget.h:412: warning: No description found for parameter 'quirk_stall_not_supp'
./include/linux/usb/gadget.h:412: warning: No description found for parameter 'quirk_zlp_not_supp'

Document those fields to make the warnings go away.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>