Since the commit below, cfg80211_chandef_dfs_required()
will warn if it gets a an NL80211_IFTYPE_UNSPECIFIED iftype
as explicitely written in the commit log.
When an virtual monitor interface is added, its type is set
in ieee80211_sub_if_data.vif.type, but not in
ieee80211_sub_if_data.wdev.iftype which is passed to
cfg80211_chandef_dfs_required() hence resulting in the
following warning: