[ALSA] sound/pci/cs46xx/: fix an off-by-one

This patch fixes an off-by-one in a snd_assert() spotted by the
Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>