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

Discovered by klocwork

Array 'iwl_rate_mcs' of size 15 may use index value(s) -1
  * rs.c:2562: index = iwl_hwrate_to_plcp_idx(rate)
    * rs.c:2562: Result of function call 'iwl_hwrate_to_plcp_idx(rate)' is '[-1,14]'
  * rs.c:2565: Array 'iwl_rate_mcs' size is 15.
  * rs.c:2565: Possible attempt to access element -1 of array 'iwl_rate_mcs'.

While at it stop using index = -1 and always use IWL_RATE_INVALID

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