Avoid multiple runs of gpgsm --learn-card

If multiple smart cards with unknown certificates are inserted then
learn cards might be triggered multiple times (once for each card). But
the gpgsm command processes all cards so that running it once is enough.

(cherry picked from commit 4ba2c3900b32f5d4761918d20984416e4e945c31)

• GnuPG-bug-id: T6846