ALSA: aoa: onyx: always initialize register read value

A lot of places in the driver use onyx_read_register() without
checking the return value, and it's been working OK for ~10 years
or so, so probably never fails ... Rather than trying to check the
return value everywhere, which would be relatively intrusive, at
least make sure we don't use an uninitialized value.

Fixes: f3d9478b ("[ALSA] snd-aoa: add snd-aoa")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>