conf: fix load_for_all_cards()

conf: fix load_for_all_cards()

The 63f7745b commit is loading the driver specific configuration
multiple times which ends with the array merges (see the bug).

Introduce the loaded compound which traces the already loaded
driver configurations and skip the multiple load requests.

Fixes: https://github.com/alsa-project/alsa-lib/issues/143
Fixes: 63f7745b ("conf: extend load_for_all_cards hook (id/value table)")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>