iwlwifi: mvm: rs: fix a potential out of bounds access

Klocwork pointed these out. There is a theoretical possibility
that rate->index might be set to IWL_RATE_INVALID (15).
This could trigger an out of bounds access on ht_vht_rates or
legacy_rates arrays. Fix it by adding some checks.

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