media: v4l2-flash-led-class: Document v4l2_flash_init() references

The v4l2_flash_init() keeps a reference to the ops struct but not to the
config struct (nor anything it contains). Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>