spcom: cleanup stale rx data when channel is closed

When rx timeout expired, and error is returned to the caller.
However, the data might arrive after the timeout expired.
As a timeout recovery (or any other error recovery), the caller
close and re-open the channel (unregister and re-register).
When a channel is closed we cleanup any stale rx data.

Change-Id: I74b8c2b1175a81976b0392b0f9e78a17622b4d84
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>