ALSA: riptide: Remove always NULL parameter

snd_riptide_pcm() takes a pointer to a pointer of a PCM where if this
parameter is provided the newly allocated PCM is stored. All callers pass
NULL though, so remove the parameter. This makes the code a bit cleaner and
shorter.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>