ALSA: dummy: Use standard macros for fixing PCM format cast

Simplify the code with the new macros for PCM format type iterations.
This fixes the sparse warnings nicely:
  sound/drivers/dummy.c:906:25: warning: restricted snd_pcm_format_t degrades to integer
  sound/drivers/dummy.c:908:25: warning: incorrect type in argument 1 (different base types)
  sound/drivers/dummy.c:908:25:    expected restricted snd_pcm_format_t [usertype] format
  sound/drivers/dummy.c:908:25:    got int [assigned] i

No functional changes, just sparse warning fixes.

Link: https://lore.kernel.org/r/20200206163945.6797-5-tiwai@suse.de


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