ALSA: nm256: Convert to generic PCM copy ops

This patch converts the nm256 driver code to use the new unified PCM
copy callback.  It's a straightforward conversion from *_user() to
*_iter() variants.

Link: https://lore.kernel.org/r/20230815190136.8987-10-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>