cfg80211: pass the actual iftype when calling cfg80211_chandef_dfs_required()

There is no need to pass NL80211_IFTYPE_UNSPECIFIED when calling
cfg80211_chandef_dfs_required() since we always already have the
interface type.  So, pass the actual interface type instead.

Additionally, have cfg80211_chandef_dfs_required() WARN if the passed
interface type is NL80211_IFTYPE_UNSPECIFIED, so we can detect
problems more easily.

Tested-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Reported-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>