iio: sw-trigger: Fix config group initialization

iio: sw-trigger: 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 trigger creation will result in undefined
behaviour when configfs is built as a mdoule since the configfs group for
the trigger is not properly initialized.

Fixes: b662f809d410 ("iio: core: Introduce IIO software triggers")
Signed-off-by: Lars-Peter Clausen <[email protected]>
Acked-by: Daniel Baluta <[email protected]>
Cc: <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>