thinkpad-acpi: don't take the first ALSA slot by default

We don't want to be the first soundcard.  We don't want to shift other
soundcards out of the way either, even if they load much later.

Ask ALSA to (by default) load us in one of the last three slots.  This
can be overriden at will using the "index" parameter.

Reported-by: Whoopie <whoopie79@gmx.net>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>