iio: adc: sc27xx: Change to polling mode to read data

On Spreadtrum platform, the headphone will read one ADC channel multiple
times to identify the headphone type, and the headphone identification is
sensitive of the ADC reading time. And we found it will take longer time
to reading ADC data by using interrupt mode comparing with the polling
mode, thus we should change to polling mode to improve the efficiency
of reading data, which can identify the headphone type successfully.

Signed-off-by: Freeman Liu <freeman.liu@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>