mailbox: tegra-hsp: Flush whole channel

The txdone can re-fill the mailbox. Keep polling the mailbox during the
flush until all the messages have been delivered.

This fixes an issue with the Tegra Combined UART (TCU) where output can
get truncated under high traffic load.

Signed-off-by: Pekka Pessi <ppessi@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Fixes: 91b1b1c3

 ("mailbox: tegra-hsp: Add support for shared mailboxes")
Cc: stable@vger.kernel.org
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>