hwmon: qpnp-adc-voltage: Update calibration logic for VADC

Clients will call adc_tm api to set thresholds for continuous monitoring
of VADC channels. VADC calibration happens at the time of first VADC
read. There is a chance that clients can call adc_tm to set thresholds
even before calibration is done. In that case thresholds are set to
zero which is not expected. Update logic such that calibration happens
either through first VADC read or while clients setting thresholds.

Change-Id: I25c8635c7ec0e140a495e4197b80e3179f60061b
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>