gpg: Fix --quick-set-expire for V5 subkey fingerprints

* g10/keyedit.c (keyedit_quick_set_expire): Use actual size of
fingerprint.

The size of the fingerprints is either 20 (V4) or 32 (V5). Using the
actual size of the fingerprints fixes the lookup of subkeys with V5
fingerprint.

• GnuPG-bug-id: T7298