iio: adc128s052: use SPDX-License-Identifier

This updates the ti-adc128s052.c file to use SPDX-License-Identifier
instead of more verbose license text.

Signed-off-by: Dan O'Donovan <dan@emutex.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>