stdio: Split out nulldev_register() and move it under #if

It's possible that NULLDEV can be disabled while it makes leftovers,
move entire device under #if.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/common/stdio.c b/common/stdio.c
index acc65ad..61fc087 100644
--- a/common/stdio.c
+++ b/common/stdio.c
@@ -28,6 +28,7 @@
 struct stdio_dev *stdio_devices[] = { NULL, NULL, NULL };
 char *stdio_names[MAX_FILES] = { "stdin", "stdout", "stderr" };
 
+#if CONFIG_IS_ENABLED(SYS_DEVICE_NULLDEV)
 static void nulldev_putc(struct stdio_dev *dev, const char c)
 {
 	/* nulldev is empty! */
@@ -44,6 +45,25 @@
 	return 0;
 }
 
+static void nulldev_register(void)
+{
+	struct stdio_dev dev;
+
+	memset(&dev, '\0', sizeof(dev));
+
+	strcpy(dev.name, "nulldev");
+	dev.flags = DEV_FLAGS_OUTPUT | DEV_FLAGS_INPUT;
+	dev.putc = nulldev_putc;
+	dev.puts = nulldev_puts;
+	dev.getc = nulldev_input;
+	dev.tstc = nulldev_input;
+
+	stdio_register(&dev);
+}
+#else
+static inline void nulldev_register(void) {}
+#endif	/* SYS_DEVICE_NULLDEV */
+
 static void stdio_serial_putc(struct stdio_dev *dev, const char c)
 {
 	serial_putc(c);
@@ -83,18 +103,7 @@
 	dev.tstc = stdio_serial_tstc;
 	stdio_register (&dev);
 
-	if (CONFIG_IS_ENABLED(SYS_DEVICE_NULLDEV)) {
-		memset(&dev, '\0', sizeof(dev));
-
-		strcpy(dev.name, "nulldev");
-		dev.flags = DEV_FLAGS_OUTPUT | DEV_FLAGS_INPUT;
-		dev.putc = nulldev_putc;
-		dev.puts = nulldev_puts;
-		dev.getc = nulldev_input;
-		dev.tstc = nulldev_input;
-
-		stdio_register(&dev);
-	}
+	nulldev_register();
 }
 
 /**************************************************************************