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>