ALSA: sh: Convert to generic PCM copy ops

This patch converts the sh_dac_audio 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/20230815190136.8987-15-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>