thermal: qcom-spmi-temp-alarm: Don't suppress negative temp

Currently driver is suppressing the negative temperature
readings from the vadc. Consumers of the thermal zones need
to read the negative temperature too. Don't suppress the
readings.

Change-Id: I147c270176d2cfce63fee71bceb64e784cfb1efa
Signed-off-by: Veera Vegivada <vvegivad@codeaurora.org>