[SERIAL] Add comment about early_serial_setup()

early_serial_setup() must not be called after console initialisation.
Add a comment prior to the function explicitly stating this.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

drivers/serial/8250.c

@@ -2326,6 +2326,12 @@ static struct uart_driver serial8250_reg = {
 	.cons			= SERIAL8250_CONSOLE,
 };
 
+/*
+ * early_serial_setup - early registration for 8250 ports
+ *
+ * Setup an 8250 port structure prior to console initialisation.  Use
+ * after console initialisation will cause undefined behaviour.
+ */
 int __init early_serial_setup(struct uart_port *port)
 {
 	if (port->line >= ARRAY_SIZE(serial8250_ports))