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

[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 <[email protected]>
Acked-by: Hans Verkuil <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>