iwlwifi: mvm: rs: use correct max expected throughput figures

The selection of the max expected throughput for a column
didn't take into account the maximal allowed rate for the current
peer. This can cause unnecessary switches during the search cycle
to columns which have no chance of beating the current throughput.

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