gpg: Fix creating on-disk subkey with on-card primary key.

* g10/keygen.c (generate_subkeypair): Ignore error code issued for
trying to verify a card based key.

We try to verify the primary key and thus seed the passphrase cache
before generating the subkey. However, the verification does not yet
work for on-card keys and thus the PASSWD --verify send to the agent
returns an error. This patch detects this error and continues without
a seeded passphrase cache. After all that pre-seeding is just a
convenience.

• GnuPG-bug-id: T3280
• Signed-off-by: Werner Koch <wk@gnupg.org>