net80211: update ieee80211_output_seqno_assign() to 802.11-2020

Update ieee80211_output_seqno_assign() to support the transmitter
sequence number assignment outlined in 802.11-2020 10.3.2.14.2
(Transmitter Requirements).

Notably this correctly assigns the QoS NULL frames a seqno outside of
the TID seqno space.

Leave stub comments for the currently supported sequence
number decisions.

Add two new functions to access and increment the sequence number space,
which will ensure that things wrap correctly. This should simplify
drivers needing to constantly invent their own methods of fetching
and incrementing the sequence number space.

Differential Revision: https://reviews.freebsd.org/D50691
Reviewed by: bz