iwlwifi: split the handler and the wake parts of the notification infra

iwlwifi: split the handler and the wake parts of the notification infra

The notification infrastructure (iwl_notification_wait_*
functions) allows to wait until a list of notifications
will come up from the firmware and to run a special handler
(notif_wait handler) when those are received.

The operation mode notifies the notification infrastructure
about any Rx being received by the mean of
iwl_notification_wait_notify() which will do two things:
1) call the notif_wait handler
2) wakeup the thread that was waiting for the notification

Typically, only after those two steps happened, the
operation mode will run its own handler for the notification
that was received from the firmware. This means that the
thread that was waiting for that notification can be
running before the operation mode's handler was called.

When the operation mode's handler is ASYNC, things get even
worse since the thread that was waiting for the
notification isn't even guaranteed that the ASYNC callback
was added to async_handlers_list before it starts to run.
This means that even calling
iwl_mvm_wait_for_async_handlers() can't guarantee that
absolutely everything related to that notification has run.
The following can happen:

Thread sending the command        Operation mode's Rx path
--------------------------        ------------------------
iwl_init_notification_wait()
iwl_mvm_send_cmd()
                                  iwl_mvm_rx_common()
                                  iwl_notification_wait_notify()
iwl_mvm_wait_for_async_handlers()
// Possibly free some data
// structure
                                  list_add_tail(async_handlers_list);
                                  schedule_work(async_handlers_wk);
                                  // Access the freed structure

Split the 'run notif_wait's handler' and the 'wake up the
thread' parts to fix this. This allows the operation mode
to do the following:

Thread sending the command        Operation mode's Rx path
--------------------------        ------------------------
iwl_init_notification_wait()
iwl_mvm_send_cmd()
                                  iwl_mvm_rx_common()
                                  iwl_notification_wait()
                                  // Will run the notif_wait's handler
                                  list_add_tail(async_handlers_list);
                                  schedule_work(async_handlers_wk);
                                  iwl_notification_notify()
iwl_mvm_wait_for_async_handlers()

This way, the waiter is guaranteed that all the handlers
have been run (if SYNC), or at least enqueued (if ASYNC)
by the time it wakes up.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>