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: b662f809 ("iio: core: Introduce IIO software triggers")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>