thermal: Modify qpnp_adc_tm driver for IIO framework

Make changes to use IIO framework APIs for reading
ADC channel information instead of VADC APIs.

Change-Id: I70ab09905d0f4a23fb5d0617ba6d031ecc7fb8bc
Signed-off-by: Manjunatha Madana <mamanj@codeaurora.org>
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>