ALSA: korg1212: Convert to the new PCM ops

Replace the copy and the silence ops with the new PCM ops.
Although we can refactor this messy code, at this time, the changes
are kept as small as possible.  Let's clean up later.

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