ASoC: blackfin: Convert to the new PCM ops

Replace the copy and the silence ops with the new PCM ops.
In AC97 and I2S-TDM mode, we need to convert back to frames, but
otherwise the conversion is pretty straightforward.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>