thermal: netlink: Add thermal_group_has_listeners() helper

Add a helper function to check if there are listeners for
thermal_gnl_family multicast groups.

For now use it to avoid unnecessary allocations and sending
thermal genl messages when there are no recipients.

In the future, in conjunction with (not yet implemented) notification
of change in the netlink socket group membership, this helper can be
used to open/close hardware interfaces based on the presence of
user space subscribers.

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>