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.

(cherry picked from commit 93d587e4d0442c7e702995b53668081240eefba2)