msm: audio: qdsp6v2: Fix for kernel crash during stability

- Userspace calling AYSN_READ during flush, READ_DONE ack for
  this is coming after FLUSH_DONE which is hitting BUG_ON as
  buffers are removed from in_queue/out_queue during flush.
- Fix is to avoid sending READ requests to DSP until flush
  is complete.

CRs-Fixed: 485288
Change-Id: Ia2a64471a3507b3f5565b695c40cc6dd9081930d
Signed-off-by: Deepa Madiregama <dmadireg@codeaurora.org>