[media] uvcvideo: Return -EACCES when trying to set a read-only control

Commit ba68c8530a263dc4de440fa10bb20a1c5b9d4ff5 (Partly revert "[media]
uvcvideo: Set error_idx properly for extended controls API failures")
also reverted part of commit 30ecb936
("uvcvideo: Return -EACCES when trying to access a read/write-only
control") by mistake. Fix it.

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>