iwlagn: free the Tx cmd when a non empty Tx queue is freed

When a non-empty Tx queueis freed, the buffer it contains must be
freed too. Since the Tx cmd are now allocated from a pool, the Tx
cmd must be freed too.

This patch avoids to destroy a non-empty pool of Tx cmd.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>