media: v4l2-dev.h: fix doc warning

Fix this warning when building the docs:

include/media/v4l2-dev.h:42: warning: Enum value 'VFL_TYPE_MAX' not described in enum 'vfl_devnode_type'
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

include/media/v4l2-dev.h

@@ -30,6 +30,7 @@
  * @VFL_TYPE_SUBDEV:	for V4L2 subdevices
  * @VFL_TYPE_SDR:	for Software Defined Radio tuners
  * @VFL_TYPE_TOUCH:	for touch sensors
+ * @VFL_TYPE_MAX:	number of VFL types, must always be last in the enum
  */
 enum vfl_devnode_type {
 	VFL_TYPE_GRABBER	= 0,