ALSA: nm256: Convert to new PCM copy ops

Replace the copy and the silence ops with the new ops.
The conversion is straightforward with standard helper functions, and
now we can drop the bytes <-> frames conversions in callbacks.

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