thinkpad-acpi: acpi_evalf fixes

Use acpi_format_exception() in acpi_evalf() instead of logging numeric
errors.

Also, when ACPICA returns an error, we should not be touching the return
object, as it is invalid.  In debug mode, acpi_evalf() callers would
printk the returned crap (but fortunately, not use it).

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>