The patch fixes the misuse of DMA channel number by Tx queue number in
iwl_tx_ctx_stop().

The problem was originally reported by Wu Fengguang who complains
iwlagn driver takes too long time when issuing `ifconfig wlan0 down`.
The patch now decreases the interface bring down time from 2 seconds
to 0.8 second.

This fixes bugs:
http://bugzilla.kernel.org/show_bug.cgi?id=11956
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1790Signed-off-by: NZhu Yi <yi.zhu@intel.com>
Tested-by: NFengguang Wu <fengguang.wu@intel.com>
Acked-by: NTomas Winkler <tomas.winkler@intel.com>
Signed-off-by: NReinette Chatre <reinette.chatre@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>