thermal: shorten too long mcast group name

[ Upstream commits 73214f5
  and f1e79e2, the latter
  adds an assertion to genetlink to prevent this from happening
  again in the future. ]

The original name is too long.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>