mac80211: don't require VHT elements for HE on 2.4 GHz

After the previous similar bugfix there was another bug here,
if no VHT elements were found we also disabled HE. Fix this to
disable HE only on the 5 GHz band; on 6 GHz it was already not
disabled, and on 2.4 GHz there need (should) not be any VHT.

Fixes: 57fa5e85 ("mac80211: determine chandef from HE 6 GHz operation")
Link: https://lore.kernel.org/r/20201013140156.535a2fc6192f.Id6e5e525a60ac18d245d86f4015f1b271fce6ee6@changeid


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