ASoC: soc-pcm: Fix playback shutdown sequence

Change closes the frontend pcm substream followed by the
backend substream to ensure correct playback shutdown
sequence.

CRs-fixed: 445499
Change-Id: Ifc6327e9b07ad40e5b4952b4db270d528b3b633b
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Meng Wang <mwang@codeaurora.org>