USB: Improve usb_fill_* documentation

Document the transfer buffer requirement. That is, the buffer must be
DMAble - otherwise data corruption might occur.

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20221220-usb-dmadoc-v4-0-74a045bf14f4@chromium.org


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