rtwn: enable seqno offload; migrate to use ieee80211_output_seqno_assign()
This should both enable the sequence number offloading and disable
the net80211 TX lock from being acquired/released/checked.
Locally tested:
- RTL8812AU, STA mode
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D52301
Differential Revision: https://reviews.freebsd.org/D50693