cfg80211/mac80211: Support disabling HE mode

Allow user to disable HE mode, similar to how VHT and HT
can be disabled.  Useful for testing.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Link: https://lore.kernel.org/r/20210204144610.25971-1-greearb@candelatech.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>