serial: core: Remove unsafe x_char optimization

uart_unthrottle() attempts to avoid sending START and the previous
x_char if the previous x_char has not yet been sent. However, this
optimization could leave the sender in a throttled state; for example,
if the sender is throttled and this unthrottle coincides with a manual
tcflow(TCION) from user-space, then neither START would be sent.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -647,12 +647,8 @@ static void uart_unthrottle(struct tty_struct *tty)
 		mask &= ~port->flags;
 	}
 
-	if (mask & UPF_SOFT_FLOW) {
-		if (port->x_char)
-			port->x_char = 0;
-		else
-			uart_send_xchar(tty, START_CHAR(tty));
-	}
+	if (mask & UPF_SOFT_FLOW)
+		uart_send_xchar(tty, START_CHAR(tty));
 
 	if (mask & UPF_HARD_FLOW)
 		uart_set_mctrl(port, TIOCM_RTS);