iwlwifi: Name fix for MPDU density for TX aggregation

Fix incorrect name for HT MPDU Density.
default set to 4 uSec

Reported-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>