Store the tx seq# of an 802.11 frame in the mbuf pkthdr; this will be
used for s/w retransmit schemes that want to access this information
w/o the overhead of decoding the raw frame. Note this also allows
drivers to record this information w/o writing the frame when the seq#
is obtained through an out-of-band mechanism (e.g. when a h/w assigned
seq# is reported in a descriptor on tx done notification).

Reviewed by: sephe, avatar