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>