iwlwifi: dynamic allocate tx queue structure

Instead of always allocate the max number of tx queue structure,
use dynamic allocation based on the number of queues in device
configuration. With these changes, device does not have to allocate more
memory than the h/w can support.

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>