ACPI thermal: Don't invalidate thermal zone if critical trip point is bad

V2: Corrected integer/long conversion.

Some BIOSes return a negative value for the critical trip point.
Especially since Windows 2006...
We currently invalidate the whole thermal zone in this case.
But it may still be needed for cooling, also without critical
trip point.

This patch invalidates the critical trip point if no _CRT
function is found or if it returns negative values, but does
not invalidate the whole thermal zone in this case.

Reference: http://bugzilla.novell.com/show_bug.cgi?id=531547

Signed-off-by: Thomas Renninger <trenn@suse.de>
Tested-by:  <clarkt@cnsp.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>