[media] v4l: Document EACCES in VIDIOC_[GS]_CTRL and VIDIOC_{G, S, TRY}_EXT_CTRLS

VIDIOC_S_CTRL and VIDIOC_S_EXT_CTRLS return EACCES when setting a read-only
control VIDIOC_TRY_EXT_CTRLS when trying a read-only control and
VIDIOC_G_CTRL and VIDIOC_G_EXT_CTRLS when getting a write-only control.
Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>