staging: iio: tsl2x7x: use IIO_CONST_ATTR for calibscale_available

The in_intensity0_calibscale_available sysfs attribute has code that
checks the device type to determine which calibration scales are
available. This check is not necessary since all of the supported
ALS device types use the scales 1 8 16 120. This patch converts the
sysfs attribute to use IIO_CONST_ATTR. The following device datasheets
were checked: tsl2571, tsl2771, tmd2771, tsl2572, tsl2772, tmd2772.

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