thermal: use integers rather than strings for thermal values

The thermal API currently uses strings to pass values to userspace. This
makes it difficult to use from within the kernel. Change the interface
to use integers and fix up the consumers.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>