mac80211: document sleep requirements for channel context ops

Channel context driver operations can sleep, so add might_sleep()
and document this.

Signed-off-by: Chaitanya T K <chaitanya.mgit@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/driver-ops.c b/net/mac80211/driver-ops.c
index 9f97343..ca1fe55 100644
--- a/net/mac80211/driver-ops.c
+++ b/net/mac80211/driver-ops.c
@@ -236,6 +236,8 @@
 	int ret = 0;
 	int i;
 
+	might_sleep();
+
 	if (!local->ops->switch_vif_chanctx)
 		return -EOPNOTSUPP;