ALSA: cs46xx: Remove always NULL parameters

The various PCM and MIDI allocation functions in this driver take a pointer
to a pointer of a PCM/MIDI object where if this parameter is provided the
newly allocated object is stored. All callers pass NULL though, so remove
the parameter. This makes the code a bit shorter and cleaner.

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