Merge get_seqnum_igtk() driver op with get_seqnum()

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.