pal: fix race condition between pcm read and ssr handling

PCM node can be closed as part of SSR offline handling,
and at the same time when pcm read comes, it can access
to null pcm node as no stream mutex is held for read.
Add stream mutex for read to avoid race condition.

Change-Id: I73b633d1ee88a7dad590556e136913c138845eaa
3 files changed