media: v4l2-dv-timings.h: convert comment into kernel-doc markup

The can_reduce_fps() is already documented, but it is not
using the kernel-doc markup. Convert it, in order to generate
documentation from it.

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