thermal: netlink: Add enum for mutlicast groups indexes

Use enum instead of hard-coded numbers for indexing multicast groups.

Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>