ALSA: snd_pcm_new api cleanup

Impact: cleanup

snd_pcm_new takes a char *id argument, although it is not modifying
the string. it can therefore be declared as const char *id.

Signed-off-by: Tim Blechmann <tim@klingt.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>