thinkpad-acpi: be more strict when detecting a ThinkPad

Use stricter checks to decide that we're running on a supported ThinkPad.
This should remove some possible false positives, although nobody ever
bothered to report any.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>