scd: Flush KDF DO (0x00F9) when it's being set.

* scd/app-openpgp.c (do_setattr): Call flush_cache_item always.

Only it was called when there was an object with KDF_DATA_LENGTH_MAX.
No matter the VALUELEN, it should be flushed.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>