scd:openpgp: Small speedup reading card properties.

* scd/app-openpgp.c (struct app_local_s): Add new flag.
(get_cached_data): Force chace use if flag is set.
(app_select_openpgp): Avoid reading DO 6E multiple times.

The do not cache property of 6E was introduced so that we can change
for example key attributes without getting into with the cache.
However, for initial reading the cache makes a lot of sense and thus we
now use this hack to only temporary cache. A better strategy would be
to clear the cache when we change card data but that is more error
prone.

• Signed-off-by: Werner Koch <wk@gnupg.org>