ALSA: sh: Convert to the new PCM ops

Replace the copy and the silence ops with the new PCM ops.
Fixed also the user-space buffer copy with the proper
copy_from_user*() variant.

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