iio: adc5: Assign calibration values

Use timer calibration as default calibration value for
ADC conversion requests. Using fresh calibration value
will increase overall conversion time for ADC channel requests.

Change-Id: I578ec77fbd5b27a1a110be960a02ef79dd8563c6
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>