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.

Cc: stable@vger.kernel.org
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Tested-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>