acpi-thermal: fix mod_timer() interval

Use relative time, not absolute.  Discovered by Jung-Ik (John) Lee
<jilee@google.com>.

Cc: Jung-Ik (John) Lee <jilee@google.com>
Acked-by: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>