ACPI: thinkpad-acpi: trivial fix of error message

Trivial fix makes the error message match the code before it (ibm->driver
vs ibm->acpi-driver) better.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>