thinkpad-acpi: disable volume control

Disable volume control by default.  It can be enabled at module load
time by a module parameter (volume_control=1).

The audio control mixer that thinkpad-acpi interacts with is fully
functional without any drivers, and operated by hotkeys.

The idea behind the console audio control is that the human operator
is the only one that can interact with it.  The ThinkVantage suite in
Windows does not allow any software-based overrides, and only does OSD
(on-screen-display) functions.

The Linux driver will, with the addition of the ALSA interface, try to
follow and enforce the ThinkVantage UI design:

The user is supposed to use the keyboard hotkeys to interact with the
console audio control.  The kernel and the desktop environment is
supposed to cooperate to provide proper user feedback through
on-screen-display functions.

Distros are urged to not to enable volume control by default.
Enabling this must be a local admin's decision.  This is the reason
why there is no Kconfig option.

Keep in mind that all ThinkPads have a normal, main mixer (AC97 or
HDA) for regular software-based audio control.  We are not talking
about that mixer here.

Advanced users are, of course, free to enable volume control and do as
they please.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Lorne Applebaum <lorne.applebaum@gmail.com>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>