random(4): Correct a bare zero to the appropriate enum

The convention for updating hc_destination[] is to index with a
random_entropy_source. Zero happens to match RANDOM_CACHED, which is
correct for this source (early random data). Spell the zero value as the
enum name instead of the magic constant.

No functional change.

Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16983