thinkpad-acpi: make volume subdriver optional

Allow the user to choose through Kconfig if the Console Audio Control
interface (aka "volume subdriver") should be available or not.

This not only saves some memory, but also allows the thinkpad-acpi
driver to be built-in even if ALSA is modular when the console audio
control interface is not wanted.

This change fixes a build problem that is causing some annoyances, in
a way that doesn't disable the entire driver on kernels without ALSA
support.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Amerigo Wang <amwang@redhat.com>
Cc: Helight Xu <helight.xu@gmail.com>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>