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>
diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index 7d35803..7c56445 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -3282,8 +3282,9 @@
 	return 0;
 }
 
-int __ieee80211_channel_switch(struct wiphy *wiphy, struct net_device *dev,
-			       struct cfg80211_csa_settings *params)
+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);
 	struct ieee80211_local *local = sdata->local;