ALSA: pcm: Minor refactoring

Make a common helper for validating the format type.
This reduces the number of cast in the code that are needed for
suppressing sparse warnings.

No functional changes, just minor refactoring.

Link: https://lore.kernel.org/r/20200206163945.6797-9-tiwai@suse.de


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