We have mgd_prepare_tx(), but sometimes drivers may want/need
to take action when the exchange finishes, whether successfully
or not.

Add a notification to the driver on completion, i.e. call the
new method mgd_complete_tx().

To unify the two scenarios, and to add more information, make
both of them take a struct that has the duration (prepare only),
subtype (both) and success (complete only).
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.5d94e78f6230.I6dc979606b6f28701b740d7aab725f7853a5a155@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>