serial: Fix send_xchar() handlers

START_CHAR() & STOP_CHAR() can be disabled if set to '\0'
(__DISABLED_CHAR).  UART drivers which define a send_xchar()
handler must not transmit __DISABLED_CHAR.

Document requirement.

Affected drivers:
sunsab
sunhv

cc: David S. Miller <davem@davemloft.net>
cc: <sparclinux@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sunhv.c

@@ -268,6 +268,9 @@ static void sunhv_send_xchar(struct uart_port *port, char ch)
 	unsigned long flags;
 	int limit = 10000;
 
+	if (ch == __DISABLED_CHAR)
+		return;
+
 	spin_lock_irqsave(&port->lock, flags);
 
 	while (limit-- > 0) {