net: lwm2m: temp_sensor: fix min/max measurement values

Initial values for the min/max measurements were 0 and this caused
issues with sensors maximums that weren't above 0 and minimums that
went below 0.  Let's update those to MAX_INT so the first sensor
value update will set those to correct values.

When resetting the measured values, let's use the current sensor
value not 0.

Signed-off-by: Michael Scott <michael.scott@linaro.org>