ALSA: riptide: postfix increment and off by one

With a postfix increment these variables are incremented beyond
CMDIF_TIMEOUT / MAX_WRITE_RETRY.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>