media: v4l2-flash-led-class.h: add kernel-doc to two helper funcs

There are two helper functions at v4l2-flash-led-class.h
that aren't documented.

Document them.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>