net: qrtr: Flush work during release

The work struct to route packets to the appropriate port may sbe active
when the endpoint unregister occurs. Flush the work struct before
freeing the node structure. This change assumes that data will not be
posted on the endpointed after getting an unregister call.

Change-Id: Ia44dff6ca339dffa3e793d1545594da960240f8e
Signed-off-by: Chris Lew <clew@codeaurora.org>