mailbox: handle empty message in tx_tick

[ Upstream commit cb710ab1 ]

We already check if the message is empty before calling the client
tx_done callback. Calling completion on a wait event is also invalid
if the message is empty.

This patch moves the existing empty message check earlier.

Change-Id: I59aac3b2c133fecf2fb9e7746fa5ec72f391288c
Fixes: 2b6d83e2 ("mailbox: Introduce framework for mailbox")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>