[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.