net80211: add support for sequence number offloading

Add support for sequence number offloading -

• Check IEEE80211_CONF_SEQNO_OFFLOAD() before doing TX lock manipulation;
• Don't call ieee80211_output_seqno_assign() if IEEE80211_CONF_SEQNO_OFFLOAD() is true.

TODO:

• this doesn't yet do beacon sequence number allocation offloading; I'll tackle that later.

Differential Revision: https://reviews.freebsd.org/D50692
Reviewed by: bz