thermal: qpnp-adc-tm: Reduce kernel log flood

Modify the pr_err log that is used for checking if
clients have a valid callback when threshold request
is made to pr_debug. Currently kernel logs are getting
flooded due to periodic request made from the client.

Change-Id: I74bf4115bc25396da16a96ad7378ba3a7375710f
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>