cfg80211: require HE capabilities for 6 GHz band

On 6 GHz band, HE capabilities must be available for all of
the interface types, otherwise we shouldn't use 6 GHz. Check
this.

Link: https://lore.kernel.org/r/20200528213443.5881cb3c8c4a.I583b54172f91f98d44af64a16c5826fe458cbb27@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>