media: uvcvideo: Fix menu count handling for userspace XU mappings

When commit 716c330 ("media: uvcvideo: Use standard names for
menus") reworked the handling of menu controls, it inadvertently
replaced a GENMASK(n - 1, 0) with a BIT_MASK(n). The latter isn't
equivalent to the former, which broke adding XU mappings from userspace.
Fix it.

Link: https://lore.kernel.org/linux-media/468a36ec-c3ac-cb47-e12f-5906239ae3cd@spahan.ch/

Cc: stable@vger.kernel.org
Reported-by: Poncho <poncho@spahan.ch>
Fixes: 716c330 ("media: uvcvideo: Use standard names for menus")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Author: pinchartl

drivers/media/usb/uvc/uvc_v4l2.c

@@ -45,7 +45,7 @@ static int uvc_control_add_xu_mapping(struct uvc_video_chain *chain,
 	map->menu_names = NULL;
 	map->menu_mapping = NULL;
 
-	map->menu_mask = BIT_MASK(xmap->menu_count);
+	map->menu_mask = GENMASK(xmap->menu_count - 1, 0);
 
 	size = xmap->menu_count * sizeof(*map->menu_mapping);
 	map->menu_mapping = kzalloc(size, GFP_KERNEL);