iio: adc: ingenic-adc: refactor ingenic_adc_read_chan_info_raw()

iio: adc: ingenic-adc: refactor ingenic_adc_read_chan_info_raw()

Extract the sample logic from ingenic_adc_read_chan_info_raw() into
a new helper function __ingenic_adc_read_chan() to improve code
readability and modularity.

The helper handles the mutex-protected section for sampling channels,
while the main function manages mutex and clock enabling/disabling.

Signed-off-by: Felipe Ribeiro de Souza <felipers@ime.usp.br>
Co-developed-by: Lucas Ivars Cadima Ciziks <lucas@ciziks.com>
Signed-off-by: Lucas Ivars Cadima Ciziks <lucas@ciziks.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>