mailbox: mediatek: implement flush function

For client driver which need to reorganize the command buffer, it could
use this function to flush the send command buffer.
If the channel doesn't be started (usually in waiting for event), this
function will abort it directly.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>