mac80211: Support adding duration for prepare_tx() callback

There are specific cases, such as SAE authentication exchange, that
might require long duration to complete. For such cases, add support
for indicating to the driver the required duration of the prepare_tx()
operation, so the driver would still be able to complete the frame
exchange.

Currently, indicate the duration only for SAE authentication exchange,
as SAE authentication can take up to 2000 msec (as defined in IEEE
P802.11-REVmd D1.0 p. 3504).

As the patch modified the prepare_tx() callback API, also modify
the relevant code in iwlwifi.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>