iwlwifi: set correct AC to swq_id for aggregation

When starting an aggregation session, the swq_id is generated in function
iwl_virtual_agg_queue_num() where the first parameter is supposed to be
the Access Class, but it used the tx fifo ID instead. This means the AC
value stored in swq_id is incorrect. To test this, look at the tx_queue
file in debugfs while transmitting Best Effort flow (ac=2), it shows:
hwq 10: read=0 write=0 stop=0 swq_id=0xa9 (ac 1/hwq 10)
After this fix, it will show:
hwq 10: read=0 write=0 stop=0 swq_id=0xaa (ac 2/hwq 10)

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>