iwlwifi: clean up rs_tx_status

Cut down on redundant code, reorganize structure, and add/improve comments.
Should contain no functional changes.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>