staging: iio: tsl2583: convert illuminance0_calibscale sysfs attr to use iio_chan_spec

The illuminance0_calibscale sysfs attribute is not currently created by
the IIO core. This patch adds the appropriate mask to iio_chan_spec,
along with the appropriate data handling in the read_raw() and
write_raw() functions, so that the sysfs attribute is created by the IIO
core. With this change, this sysfs entry will have its prefix changed
from illuminance0_ to in_illuminance_.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>