staging: iio: tsl2x7x: convert in_proximity0_calibscale_available to use IIO_CONST_ATTR

The sysfs attribute in_proximity0_calibscale_available is currently
created by using DEVICE_ATTR_RO(). Convert this over to use
IIO_CONST_ATTR().

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