ASoC: DPCM: Remove redundant code introduced during framework update

There is redundant code introduced during ALSA DPCM update to add
support for compressed audio, which calls the soc_pcm_prepare()
twice. Update to remove the reduandant code.

Change-Id: Id4ce3500e1bbbd614c637b43532c4dc99b13435e
CRs-fixed: 672001
Signed-off-by: Kuirong Wang <kuirongw@codeaurora.org>