staging: iio: tsl2x7x: migrate in_illuminance0_integration_time sysfs attribute to iio_chan_spec

The driver explicitly creates the in_illuminance0_integration_time sysfs
attribute outside the IIO core. This attribute is available in the IIO
core so this patches migrates the attribute to be created by
the iio_chan_spec.

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