usb: gadget: add doc to struct usb_composite_dev

Added documentation to new struct members for WebUSB:
* bcd_webusb_version
* b_webusb_vendor_code
* landing_page
* use_webusb
to avoid warnings in the build of htmldocs

Fixes: 93c47394 ("usb: gadget: add WebUSB landing page support")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jó Ágila Bitsch <jgilab@gmail.com>
Link: https://lore.kernel.org/r/Y95MRZZz3yC5lETB@jo-einhundert


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>