tty: serial: men_z135_uart: Wakeup UART after transmitting

Call uart_write_wakeup() after writing the hardware FIFO and updateing the FIFO
pointers.

This fixes high latency and jitter on PPP over Serial links.
Reported-by: NJun Shih <Jun.Shih@pason.com>
Tested-by: NJun Shih <Jun.Shih@pason.com>
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@men.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/men_z135_uart.c

@@ -308,9 +308,6 @@ static void men_z135_handle_tx(struct men_z135_port *uart)
 	if (port->x_char)
 		goto out;
 
-	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
-		uart_write_wakeup(port);
-
 	/* calculate bytes to copy */
 	qlen = uart_circ_chars_pending(xmit);
 	if (qlen <= 0)
@@ -357,6 +354,9 @@ static void men_z135_handle_tx(struct men_z135_port *uart)
 
 	port->icount.tx += n;
 
+	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
+		uart_write_wakeup(port);
+
 irq_en:
 	if (!uart_circ_empty(xmit))
 		men_z135_reg_set(uart, MEN_Z135_CONF_REG, MEN_Z135_IER_TXCIEN);