scd: Use lock_slot for apdu_send_direct.

* scd/apdu.c (apdu_send_direct): Use lock_slot.

With trylock_slot, it may return SW_HOST_BUSY. This may occur when
apdu_get_status is called by scd_update_reader_status_file.

Simply using lock_slot is much easier for user of apdu_send_direct.

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