iwlwifi: don't allow the FW to return invalid ch indices

If the FW returns an invalid channels count in response to an MCC request,
make sure we don't reference invalid indices in the channels array.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>