mac80211: fix sparse warning caused by __ieee80211_channel_switch()

Commit 59af6928 (mac80211: fix CSA tx queue stopping) introduced a
sparse warning:

net/mac80211/cfg.c:3274:5: warning: symbol '__ieee80211_channel_switch' was not declared. Should it be static?

Fix it by declaring the function static.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/cfg.c

@@ -3282,7 +3282,8 @@ static int ieee80211_set_csa_beacon(struct ieee80211_sub_if_data *sdata,
 	return 0;
 }
 
-int __ieee80211_channel_switch(struct wiphy *wiphy, struct net_device *dev,
+static int
+__ieee80211_channel_switch(struct wiphy *wiphy, struct net_device *dev,
 			   struct cfg80211_csa_settings *params)
 {
 	struct ieee80211_sub_if_data *sdata = IEEE80211_DEV_TO_SUB_IF(dev);