Move all the life cycle of the name to add_mapping. This simplifies
the error handling inside uvc_ioctl_ctrl_map and solves a memory leak
when kemmdup fails.

Also make sure that for custom controls, the user provides a valid name.

Fixes: 07adedb5c606 ("media: uvcvideo: Use control names from framework")
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>