iwlwifi: 3945 split tx_complete to command and packet function

This patch
1. removes cmd completion from iwl3945_tx_queue_reclaim
and creates iwl3945_cmd_queue_reclaim.
1. removes 11n relevant elements from this function
2. removes call to ieee80211_tx_status_irqsafe
and uses ieee80211_tx_status only

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