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: NLuciano Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>