thinkpad-acpi: fix ALSA callback return status

Clemens Ladisch reports that thinkpad-acpi improperly implements the
ALSA API, and always returns 0 for success for the "put" callbacks
while the API requires it to return "1" when the control value has
been changed in the hardware/firmware.

Rework the volume subdriver to be able to properly implement the ALSA
API.  Based on a patch by Clemens Ladisch <clemens@ladisch.de>.

This fix is also needed on 2.6.33.

Reported-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: stable@kernel.org