iwlwifi: TX setup fix confusion between TX queue and TX DMA channel

This patch configures correctly TX DMA channel. It is not
the same as TX queue.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>