mac80211: scan: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The use of "rcu_assign_pointer()" is NULLing out the pointer.
According to RCU_INIT_POINTER()'s block comment:
"1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
it is better to use it instead of rcu_assign_pointer() because it has a
smaller overhead.

The following Coccinelle semantic patch was used:
@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
  (..., NULL)
Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/scan.c

@@ -1094,7 +1094,7 @@ int ieee80211_request_sched_scan_stop(struct ieee80211_sub_if_data *sdata)
 	if (rcu_access_pointer(local->sched_scan_sdata)) {
 		ret = drv_sched_scan_stop(local, sdata);
 		if (!ret)
-			rcu_assign_pointer(local->sched_scan_sdata, NULL);
+			RCU_INIT_POINTER(local->sched_scan_sdata, NULL);
 	}
 out:
 	mutex_unlock(&local->mtx);