iio: sw-device: Fix config group initialization

Use the IS_ENABLED() helper macro to ensure that the configfs group is
initialized either when configfs is built-in or when configfs is built as a
module. Otherwise software device creation will result in undefined
behaviour when configfs is built as a module since the configfs group for
the device not properly initialized.

Similar to commit b2f0c09664b7 ("iio: sw-trigger: Fix config group
initialization").

Fixes: 0f3a8c3f34f7 ("iio: Add support for creating IIO devices via configfs")
Reported-by: Miguel Robles <miguel.robles@farole.net>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Daniel Baluta <daniel.baluta@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/sw_device.h

@@ -62,7 +62,7 @@ void iio_swd_group_init_type_name(struct iio_sw_device *d,
 				  const char *name,
 				  struct config_item_type *type)
 {
-#ifdef CONFIG_CONFIGFS_FS
+#if IS_ENABLED(CONFIG_CONFIGFS_FS)
 	config_group_init_type_name(&d->group, name, type);
 #endif
 }