drivers: mailbox: retry message submission

msg_submit() bails if there are no requests in the mailbox queue or if
there is an valid active request that is pending a tx_done response. If
the controller had returned -EAGAIN when trying to send data, the
framework should unlock the spinlock and retry sending the request to
the controller.

Change-Id: I05cd60730b010a8018a3e6107fd340ad8c250fd7
Signed-off-by: Lina Iyer <ilina@codeaurora.org>