scd:openpgp: Avoid PIN caching issues after re-select.

* scd/app-openpgp.c (do_reselect): Clear PIN cache flags.

It seems that the verification status of the OpenPGP app on a Yubikey
is reset on a select. We need to reflect this in our cache to avoid a
"Bad PIN" error on computing a signature.

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