ALSA: cs4281: Remove always NULL parameters

snd_cs4281_pcm() and snd_cs4281_midi() 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>