iwl3945: set TFD_QUEUE_MAX to correct value

Total number of queues is 8 but only 7 of them are TX queues.
4 AC(Data) queue ,1 CMD and 2 HCCA. The HCCA queues are not used.
max_txq_num is set to maximum usable TX queues.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>