ANDROID: GKI: 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.

Signed-off-by: Lina Iyer <ilina@codeaurora.org>
Change-Id: Id58c7365be8c6bfc7f90fe9445c88c1246d2d7f8
(cherry picked from commit e5c2b83b3daae99cfd3ef55b2c132e8bf4c22b27)
Bug: 155339628
Signed-off-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>