cfg80211: don't allow drivers to unset NL80211_FEATURE_SCAN_FLUSH

As the flag is entirely implemented in cfg80211, it should
have been a global feature flag (which I believe didn't
exist at the time). However, there's no reason to allow
drivers to unset the flag, so don't allow it and remove
the validation of NL80211_SCAN_FLAG_FLUSH.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/wireless/core.c b/net/wireless/core.c
index aff959e..9dca092 100644
--- a/net/wireless/core.c
+++ b/net/wireless/core.c
@@ -357,8 +357,6 @@
 	rdev->wiphy.rts_threshold = (u32) -1;
 	rdev->wiphy.coverage_class = 0;
 
-	rdev->wiphy.features = NL80211_FEATURE_SCAN_FLUSH;
-
 	return &rdev->wiphy;
 }
 EXPORT_SYMBOL(wiphy_new);
@@ -566,6 +564,8 @@
 	/* check and set up bitrates */
 	ieee80211_set_bitrate_flags(wiphy);
 
+	rdev->wiphy.features |= NL80211_FEATURE_SCAN_FLUSH;
+
 	rtnl_lock();
 	res = device_add(&rdev->wiphy.dev);
 	if (res) {