serial_core: Fix conditional start_tx on ring buffer not empty

If the serial_core ring buffer empties just as the tty layer receives
an XOFF, then start_tx will never be called when the tty layer
receives an XON as the serial_core ring buffer is empty.  This will
possibly leave a few bytes trapped in the fifo for drivers that
disable the transmitter when flow controlled.
Signed-off-by: NSeth Bollinger <sethb@digi.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -89,8 +89,7 @@ static void __uart_start(struct tty_struct *tty)
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port = state->uart_port;
 
-	if (!uart_circ_empty(&state->xmit) && state->xmit.buf &&
-	    !tty->stopped && !tty->hw_stopped)
+	if (!tty->stopped && !tty->hw_stopped)
 		port->ops->start_tx(port);
 }