hdspmixer: Fix ALSA snd_ctl_open error when running with three cards.

hdspmixer: Fix ALSA snd_ctl_open error when running with three cards.

If three (or more) RME cards are installed in one box, hdspmixer will
try to open a non-existing 4th card, causing an error in snd_ctl_open
and finally terminates itself.

cards[] is a static array, and one must not read beyond the last
element. The solution is far from elegant, however, it's a rather
unintrusive change.

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