[media] uvcvideo: Set error_idx properly for extended controls API failures

When one of the requested controls doesn't exist the error_idx field
must reflect that situation. For G_EXT_CTRLS and S_EXT_CTRLS, error_idx
must be set to the control count. For TRY_EXT_CTRLS, it must be set to
the index of the unexisting control.
This issue was found by the v4l2-compliance tool.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>