staging: iio: tsl2x7x: don't setup event handlers if interrupts are not configured

The driver would expose to userspace the events directory even if the
interrupts were not configured. This patch changes the driver so that
the events directory is not exposed to user space if interrupts are
not configured. This patch also corrects the indentation of the
chan_table_elements and info structure members.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>