drivers/platform/x86/thinkpad_acpi.c: don't test unsigned int for negativity

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80231


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>