iwlwifi: Add comments to some driver data structures

Add comments to some driver data structures

Remove unused "sched_retry" member from struct iwl3945_tx_queue

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>