mac80211: accept aggregation sessions on 6 GHz

On 6 GHz, stations don't have ht_supported set, but they can
still do aggregation since they must have HE, allow that.

Link: https://lore.kernel.org/r/20200528213443.776d3c891b64.Ifa099d450617b50c691832b3c4aa08959fab520a@changeid


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