media: v4l2-subdev: document controls need _FL_HAS_DEVNODE

Control events can be subscribed and received by the user. Therefore
drivers that support controls must expose the
V4L2_SUBDEV_FL_HAS_EVENTS flag.

[As discussed in https://lkml.org/lkml/2018/11/27/637]

Reported-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

include/media/v4l2-subdev.h

@@ -776,7 +776,11 @@ struct v4l2_subdev_internal_ops {
 #define V4L2_SUBDEV_FL_IS_SPI			(1U << 1)
 /* Set this flag if this subdev needs a device node. */
 #define V4L2_SUBDEV_FL_HAS_DEVNODE		(1U << 2)
-/* Set this flag if this subdev generates events. */
+/*
+ * Set this flag if this subdev generates events.
+ * Note controls can send events, thus drivers exposing controls
+ * should set this flag.
+ */
 #define V4L2_SUBDEV_FL_HAS_EVENTS		(1U << 3)
 
 struct regulator_bulk_data;