scd: Fix for inactive card at start by internal CCID driver.

* scd/ccid-driver.c (do_close_reader): Set NULL on close.
(bulk_in): Move DEBUGOUT and check by EP_INTR.
(ccid_get_atr): Clear powered_off flag after initial status check.

Many card readers automatically turn on inserted card, but some
defaults to turning off at start.

• GnuPG-bug-id: T3508
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>