media: uvcvideo: Avoid returning invalid controls

commit 414d3b49 upstream.

If the memory where ctrl_found is placed has the value of uvc_ctrl and
__uvc_find_control does not find the control we will return an invalid
index.

Fixes: 6350d6a4 ("media: uvcvideo: Set error_idx during ctrl_commit errors")
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>