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 b2f0c096 ("iio: sw-trigger: Fix config group
initialization").

Fixes: 0f3a8c3f ("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>