iwlwifi: support DSSS/CCK mode in 40 MHz

All hardware after 4965 supports this. It's likely that
it wasn't set because for 4965 it was irrelevant (HT is
only supported on 5 GHz there) and then never updated.

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