ALSA: xen-front: Refine indentations and constify snd_pcm_ops

snd_pcm_ops are not supposed to change. So mark the
non-const structs as const. Also, refine indentation
to ncrease readability.

Signed-off-by: Nick Simonov <nicksimonovv@gmail.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>