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: I5b3b7bc005816141537434be47ec743c7c02b274
CRs-fixed: 672001
Signed-off-by: Kuirong Wang <kuirongw@codeaurora.org>