iwlwifi: mvm: rs: repeat initial legacy rates in LQ table

Repeating the legacy rates avoids degrading quickly to
lower rates due to collisions which is common when doing
TCP Tx traffic in legacy.
This slightly improves TCP Tx throughput while working
in legacy in different scenarios.

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