thermal: adc-tm5: Fix pointer dereference error

Fix pointer dereference and uninitialized element errors
during driver initialization and when client sets
temperature thresholds.

Change-Id: Ia1b1eb263f8468c096210a0ccbe2098b4a70f547
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>