iwlwifi: mvm: rs: fix a theoretical access to uninitialized array elements

Klocwork is unhappy as ht_vht_rates might be accessed with
rate->index being set to values between 0 and 3 which will
lead to accessing uninitialized array elements. Effectively this
doesn't happen as in HT/VHT we're not using these rate indices.
Still fix this.

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