rpmsg: smd: Use announce_create to process any receive work

It is possible that incoming data arrives before the client driver has
reached a point in the probe method where adequate context for handling
the incoming message has been established.

In the event that the client's callback function returns an error the
message will be left on the FIFO and by invoking the receive handler
after the device has been probed the message will be picked off the FIFO
and the callback invoked again.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>