iwlwifi: add aggregation tables to the rate scaling algorithm

Current rate scale algorithm fluctuates between different MIMO modes fairly
rapidly, causing widely varying performance. These fluctuations occur because in
the rate_scale tables for expected throughput the values are not very different
for different modes.

However, when aggregation is turned on and MAC overhead is reduced, the
expected throughput for different MIMO modes grows and different modes have
vastly different performance. Add expected throughput tables for this case.

We also need to keep track of aggregation status per-station, so we add the
"is_agg" field to struct lq_sta.

Also includes cleanup of comments and variable names in/around the affected
code.
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>