iwlwifi: mvm: rs: overhaul rs_get_best_rate

rs_get_best_rate determines the optimal rate to try in a
new Tx column. Currently we were sometimes trying a too high
rate which would lead us to fail and avoid switching to the
new column despite it having a potential to be better.
Change the logic to try and find the first rate which would
exceed the current actual throughput or be more aggressive
if the success ratio is good.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>