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

commit 6d00f4ec upstream.

When commit 716c3304

 ("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: 716c3304

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