[ALSA] sound: au88x0_pcm.c fix integer as NULL pointer warning

sound/pci/au88x0/au88x0_pcm.c:508:15: warning: Using plain integer as NULL pointer

Also some small codingstyle fixes.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>