scd: Fix receive buffer size.

* scd/apdu.c (send_le): Fix the size, adding two for status
bytes to Le.

This is long standing bug. So far, Le was not exact value.
Since forthcoming change will introduce exact value of expected length
of response data, this change is needed.

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