LinuxKPI: 802.11: improve prep_tx_info

Over time struct ieee80211_prep_tx_info has grown further fields.
One which is becoming mandatory is the subtype (of the mgmt frame).
iwlwifi(mld) has a WARN for it if it does not match, so we now have
to set this for proper operation. In addition we are tyring to improve
the situation of setting/unsetting (prepare_tx/complete_tx) in various
states and cleanup the use of other fields but link_id which we now
leave as a marker for the future everywhere.
The general problem we are facing is that our hook surface in this case
is the state machine but likely would have to be tx/rx mgmt frames but
we would alos have to driver the TX queues from there which is tricky.
The long-term answer is to change net80211.

Further the hardware flag DEAUTH_NEED_MGD_TX_PREP is dead and was
removed again in favour of leting drivers deal with it. iwlwifi(mvm)
likely being the only driver which ever used this.

Sponsored by: The FreeBSD Foundation
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