iwlwifi: mvm: rs: fix mapping from HT/VHT rates to legacy

The table rs_ht_to_legacy is used to get the next legacy rate
following the last HT or VHT rate in the LQ rates table.
The mapping wasn't correct as well as didn't include entries
for MCS8/9 which led to out of bounds access. This didn't
trigger a crash but led to legacy rate entries using 1Mbps
rate.
In 5Ghz this probably caused the Tx to fail completely given
that Tx attempt would have reached the legacy entries and 1Mbps
isn't valid.

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