ASoC: msm: Fix issue with Pause/resume

The driver is sending the next buffer everytime after coming
out of PAUSE. This shouldnt happen as the previous driver might
still be with DSP.

Fix by putting appropriate check to the pending_buffer flag.
The pending_buffer flag should also be atomic to handle
intermediate close of session.

Change-Id: Ieb102b968b6d18736bab0cf4298cc51b7ed3affe
CRs-Fixed: 326259
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>