thinkpad_acpi: use DEVICE_ATTR_* macros

Use the DEVICE_ATTR_* macros to reduce boiler plate.

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>