tty/serial: atmel: fix race condition (TX+DMA)

If uart_flush_buffer() is called between atmel_tx_dma() and atmel_complete_tx_dma(), the circular buffer has been cleared, but not atmel_port->tx_len.
That leads to a circular buffer overflow (dumping (UART_XMIT_SIZE - atmel_port->tx_len) bytes).

---
NB: this will apply on kernels 4.3+, for kernels before 4.3, the code will be:
sg_dma_len(&atmel_port->sg_tx) = 0;

 drivers/tty/serial/atmel_serial.c | 5 +++++
 1 file changed, 5 insertions(+)

index d9c05e05d896..5c23fb4e552b 100644
@@ -1954,6 +1954,11 @@ static void atmel_flush_buffer(struct uart_port *port)
 		atmel_uart_writel(port, ATMEL_PDC_TCR, 0);
 		atmel_port->pdc_tx.ofs = 0;
 	}
+	/*
+	 * in uart_flush_buffer(), the xmit circular buffer has just
+	 * been cleared, so we have to reset tx_len accordingly.
+	 */
+	atmel_port->tx_len = 0;
 }
 
 /*