hdspmixer: Add support for RME AIO AEB boards

AIO allows to connect Analogue Extension Boards (4in/4out).

Provide those channels in hdspmixer, too.

The channel maps were copied from the driver, hence the changed casing
in the comments.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>