iio: Fix unitialized use of list

The dev_attr list is initialized in __iio_add_event_config_attrs which
is called only when indio_dev->channels is true. Nevertheless the list
is used unconditionally later in iio_device_register_eventset which
results in a NULL pointer exception. To fix this unconditionally
initialize the list in iio_device_register_eventset.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>