Make these const as they are only passed to a const argument of the
function snd_pcm_set_ops in the file referencing them. Also, add const
to the declaration in the headers.

Structures found using Coccinelle and changes done by hand.
Signed-off-by: NBhumika Goyal <bhumirks@gmail.com>
Reviewed-by: NTakashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>