hwmon: qpnp-adc-current: Add init_completion

Initialize the completion before requesting a
conversion. This is to avoid cases where we might
miss the completion and rely on the timed wait which
would cause slight delay before returning the result
to the client.

Change-Id: I49b635699c3432ca8363d68493ea02e17f7c6c64
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>