api: Use hashtable function for API_env_enum

The current code can loop undefinitly as it doesn't parse
correctly the env data.
Since the env is an hashtable, use the hashtable function for
the API_ENV_ENUM api call.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed-by: Simon Glass <sjg@chromium.org>