hwmon: qpnp-adc: Add check for channel selection for PMI632

There is a possibility of writes to the ADC channel selection
register not working as expected on PMI632. Read back and
check the written value.

Change-Id: I123387638413f81e70fe74a1ea23fff5c2403a8c
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>