serial: 8250_platform: Explicitly show we initialise ISA ports only once

serial8250_isa_init_ports() uses home grown approach to make itself
a singleton. Instead, explicitly show that we initialise ISA ports
once by providing a helper function which calls the original function
via DO_ONCE() macro.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240606184841.3102545-1-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_platform.c

@@ -9,6 +9,7 @@
 #include <linux/array_size.h>
 #include <linux/module.h>
 #include <linux/moduleparam.h>
+#include <linux/once.h>
 #include <linux/platform_device.h>
 
 #include <linux/serial_8250.h>
@@ -53,15 +54,10 @@ void serial8250_set_isa_configurator(serial8250_isa_config_fn v)
 }
 EXPORT_SYMBOL(serial8250_set_isa_configurator);
 
-void __init serial8250_isa_init_ports(void)
+static void __init __serial8250_isa_init_ports(void)
 {
-	static int first = 1;
 	int i, irqflag = 0;
 
-	if (!first)
-		return;
-	first = 0;
-
 	if (nr_uarts > UART_NR)
 		nr_uarts = UART_NR;
 
@@ -102,6 +98,11 @@ void __init serial8250_isa_init_ports(void)
 	}
 }
 
+void __init serial8250_isa_init_ports(void)
+{
+	DO_ONCE(__serial8250_isa_init_ports);
+}
+
 /*
  * Register a set of serial devices attached to a platform device.  The
  * list is terminated with a zero flags entry, which means we expect