iwlwifi: 7000: disable HT greenfield support

The 7000 series devices don't support HT greenfield mode
so don't advertise or use it.

Signed-off-by: Beni Lev <beni.lev@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>