[media] uvcvideo: Set error_idx properly for S_EXT_CTRLS failures

The uvc_set_ctrl() calls don't write to the hardware. A failure at that
point thus leaves the device in a clean state, with no control modified.
Set the error_idx field to the count value to reflect that, as per the
V4L2 specification.
TRY_EXT_CTRLS is unchanged and the error_idx field must always be set to
the failed control index in that case.

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>