Modules: au88x0 driver

since idx is used as an index for vortex_pcm_prettyname[VORTEX_PCM_LAST],
it should not be equal to VORTEX_PCM_LAST. This fixes coverity bug id #572
Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>