wifi: iwlwifi: add a few rate index validity checks

Validate index before access iwl_rate_mcs to keep rate->index
inside the valid boundaries. Use MCS_0_INDEX if index is less
than MCS_0_INDEX and MCS_9_INDEX if index is greater then
MCS_9_INDEX.

Signed-off-by: Anjaneyulu <pagadala.yesu.anjaneyulu@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614123447.79f16b3aef32.If1137f894775d6d07b78cbf3a6163ffce6399507@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>