acpi thermal: fix result check

thermal_zone_device_register() uses the ERR_PTR macro on its return values.  A
correct check is to use the IS_ERR() macro.

The 2.6.25 kernels panic on Compaq AP550 without this patch as it has more
then 10 (THERMAL_MAX_TRIPS) trip points (there are 12).

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Len Brown <lenb@kernel.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>