The audioformat is referred in many places but most of usages are
read-only.  Let's add const prefix in the possible places.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-28-tiwai@suse.de


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