ALSA: pcm: More constifications

Apply const prefix to more possible places: the string tables for PCM
format and co, the table for the PCM type helpers, etc.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-6-tiwai@suse.de


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