cfg80211: regulatory: use RCU_INIT_POINTER

rcu_assign_pointer() ensures that the initialization of a structure is
carried out before storing a pointer to that structure. However, in the
case that NULL is assigned there's no structure to initialize so using
RCU_INIT_POINTER instead is safe and more efficient.
Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
[rewrite commit log]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/reg.c

@@ -2594,7 +2594,7 @@ void wiphy_regulatory_deregister(struct wiphy *wiphy)
 		reg_num_devs_support_basehint--;
 
 	rcu_free_regdom(get_wiphy_regdom(wiphy));
-	rcu_assign_pointer(wiphy->regd, NULL);
+	RCU_INIT_POINTER(wiphy->regd, NULL);
 
 	if (lr)
 		request_wiphy = wiphy_idx_to_wiphy(lr->wiphy_idx);