scd: Fix CCID internal driver for interrupt transfer.

* scd/ccid-driver.c (intr_cb): Handle the case of multiple messages.

Backport master commit of:

7cbb513a2dc150a90a30c53316970df2a439d494

SPR532 USB Smart Card Reader (also know as SPR332) may send two
messages at once for a single interrupt transfer. An example transfer
observed was like: 50 03 50 02, which is considered valid, according
to the CCID specification.

• GnuPG-bug-id: T5065
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>