ALSA: line6: make snd_pcm_ops const

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: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>