serial: 8250: Do nothing if nr_uarts=0

When nr_uarts was set to 0 (via config or 8250_core.nr_uarts), we crash
early on x86 because serial8250_isa_init_ports dereferences base_ops
which remains NULL. In fact, there is nothing to do for all the callers
of serial8250_isa_init_ports if there are no uarts.

Based on suggestions by Peter Hurley.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -3535,6 +3535,9 @@ static struct console univ8250_console = {
 
 static int __init univ8250_console_init(void)
 {
+	if (nr_uarts == 0)
+		return -ENODEV;
+
 	serial8250_isa_init_ports();
 	register_console(&univ8250_console);
 	return 0;
@@ -3565,7 +3568,7 @@ int __init early_serial_setup(struct uart_port *port)
 {
 	struct uart_port *p;
 
-	if (port->line >= ARRAY_SIZE(serial8250_ports))
+	if (port->line >= ARRAY_SIZE(serial8250_ports) || nr_uarts == 0)
 		return -ENODEV;
 
 	serial8250_isa_init_ports();
@@ -3932,6 +3935,9 @@ static int __init serial8250_init(void)
 {
 	int ret;
 
+	if (nr_uarts == 0)
+		return -ENODEV;
+
 	serial8250_isa_init_ports();
 
 	printk(KERN_INFO "Serial: 8250/16550 driver, "