libcacard: correct T0 historical bytes size
libcacard: correct T0 historical bytes size

The VCARD_ATR_PREFIX macro adds a prefix of 6 characters only.

pcsc_scan was complaining before the patch:

+ Historical bytes: 56 43 41 52 44 5F 4E 53 53
 ERROR! ATR is truncated: 2 byte(s) is/are missing

Signed-off-by: Marc-André Lureau <f20011f729ce65c6b731da6d94357379565098f1@redhat.com>
Reviewed-by: Alon Levy <816c907b754fa7b5d4bb5eb91e0fc01e5510e64d@redhat.com>

SPICE/libcacard,elmarco/libcacard,elmarco/libcacard,SPICE/libcacard