mm-audio: Fix openmax encoder hang during flush

Flush issued by command thread should unblock the read/write calls
in openmax component.
There is race condition between input/output thread and command
thread in openmax component due to which read/write is not unblocked
which results in a hang.
This change makes command thread to issue multiple flush when it is
wait for ack to avoid a hang.

Change-Id: Ia4486f2e53f56a8413c2115b7f418b29b4bc54c5
CRs-fixed: 466764