hwmon: qpnp-adc-voltage: Add timeout for end of conversion

Add a timeout while the ADC read API waits for the end of
conversion interrupt. After a timeout, the status register
is read to ensure the ADC conversion is complete. The
timeout is added to return back to the user if the end
of conversion interrupt is not triggered.

Change-Id: I0deed0099212221c938a5ab7f775231e2bc85905
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>