ALSA: usb-audio: Use pcm_for_each_format() macro for PCM format iterations

The new macro can fix the sparse warnings gracefully:
  sound/usb/proc.c:73:31: warning: restricted snd_pcm_format_t degrades to integer
  sound/usb/proc.c:73:38: warning: restricted snd_pcm_format_t degrades to integer
  sound/usb/proc.c:73:61: warning: restricted snd_pcm_format_t degrades to integer

No functional changes, just sparse warning fixes.

Link: https://lore.kernel.org/r/20200206163945.6797-4-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>