Merge get_seqnum_igtk() driver op with get_seqnum()

IEEE 802.11w uses distinct key indexes (4 and 5) so the same
get_seqnum() handler can be used to fetch packet number for both
TKIP/CCMP and BIP(using IGTK).

Since the new get_seqnum_igtk() handler was not actually implemented by
any driver wrapper, this may also fix BIP/IGTK sequence number reporting
with driver_nl80211.c.

src/drivers/driver_ndis.c

@@ -3211,7 +3211,6 @@ const struct wpa_driver_ops wpa_driver_ndis_ops = {
 	NULL /* set_ieee8021x */,
 	NULL /* set_privacy */,
 	NULL /* get_seqnum */,
-	NULL /* get_seqnum_igtk */,
 	NULL /* flush */,
 	NULL /* set_generic_elem */,
 	NULL /* read_sta_data */,