This makes sure that during a mailbox transfer no other mailbox transfer
is started. This happens on a raspberrypi3 where one GPIO on the
firmware GPIO controller (which is controlled via mailbox) is registered
as a heartbeat LED. With this it can happen that during a mailbox
transfer the poller which drives the heartbeat LED is triggered in
is_timeout() which results in corrupted transfers.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>