mailbox: Export mbox_flush()

The mbox_flush() function can be used by drivers that are built as
modules, so the function needs to be exported.

Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>