media: media-ioc-enum-links.rst: fix incorrect reserved field documentation

The reserved field array for struct media_link_desc has length 2, not 4.

And the reserved field array of struct media_links_enum was never documented
at all.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>