drivers: mailbox: fix race resulting in multiple message submission

The mailbox sends a request to the controller and the tx_done callback
received for that request clears the active_req pointer. The callback
sends the next request in the queue, if there is one. When a controller
is busy and cannot accept any more requests until the interrupt is
cleared, it would return -EAGAIN. The mailbox controller must unlock its
spinlock and retry again.

Change-Id: Id58c7365be8c6bfc7f90fe9445c88c1246d2d7f8
Signed-off-by: Lina Iyer <ilina@codeaurora.org>