ALSA: gus: Convert to the new PCM ops

Replace the copy and the silence ops with the new PCM ops.
For simplifying the code a bit, two local helpers are introduced here:
get_bpos() and playback_copy_ack().

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