iwlwifi: disable greenfield transmissions as a workaround

There's a bug that causes the rate scaling to get stuck
when it has to use single-stream rates with a peer that
can do GF and SGI; the two are incompatible so we can't
use them together, but that causes the algorithm to not
work at all, it always rejects updates.

Disable greenfield for now to prevent that problem. The
MVM driver currently only works on devices that don't
support greenfield anyway, but better be safe and not
allow us to forget about this.

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>