iio: adc: Update QCOM ADC license to SPDX format

Update QCOM ADC header file to SPDX format.

Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>