ALSA: xen: Convert to generic PCM copy ops

This patch converts the xen frontend driver code to use the new
unified PCM copy callback.  It's a straightforward conversion from
*_user() to *_iter() variants.

Note that copy_from/to_iter() returns the copied bytes, hence the
error condition is adjusted accordingly.

Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Link: https://lore.kernel.org/r/20230815190136.8987-16-tiwai@suse.de


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