thinkpad-acpi: adopt input device

Properly init the parent field of the input device.  Thanks to Alan
Jenkins, who noted this problem in a different driver.

Reported-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>