rpmsg: glink: spi: Ensure rx_done is sent before data

The WDSP processor has limited memory for intents. It is important that
intents are returned before sending additional requests. This change
removes the rx_intent workqueue and sends the rx done cmd while holding
the intent lock to prevent sends from happening before the rx done cmd
is sent.

Send the rx done cmd before calling the client's rx cb. This ensures
the spi send work is done before the client can signal its next send.
Free the local intent after the the rx cb. This sequence should reduce
the intent lock contention.

Change-Id: Icb91b5ab9171275a4ed1cb0fffd70dab11d9f526
Signed-off-by: Chris Lew <clew@codeaurora.org>