serial: 8250: remove return statements from void function

serial8250_set_mctrl() is a void type function.  Returning something
does not look nice.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -2006,8 +2006,9 @@ EXPORT_SYMBOL_GPL(serial8250_do_set_mctrl);
 static void serial8250_set_mctrl(struct uart_port *port, unsigned int mctrl)
 {
 	if (port->set_mctrl)
-		return port->set_mctrl(port, mctrl);
-	return serial8250_do_set_mctrl(port, mctrl);
+		port->set_mctrl(port, mctrl);
+	else
+		serial8250_do_set_mctrl(port, mctrl);
 }
 
 static void serial8250_break_ctl(struct uart_port *port, int break_state)