mac80211: don't allow 40MHz tx rates in case of 20MHz chandef

When 20MHz chandef is used, 40MHz rates shouldn't be
used (by the rate-control algorithm), even if the sta
ht capabilities indicate support for it.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Singed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>