iio: adc5: 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: I502d40843e8677837a019dff3b3e13ac819b6074
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>