Dont check for temperatures in /thermal_zone where mode=disabled

[Saumya40-codes]

Relevant issue

Fixes: https://github.com/prometheus/node_exporter/issues/2980

Current behaviour

• node_exporter checks for the contents of temp file even when mode is disabled due to which in the case when temp file contains a non desired value the thermal_zone collector fails and no longer gives its related metrics

Proposed solution

• Have an early exit and dont check rest if mode is set to disabled

Testing

• replacing github.com/prometheus/procfs in node_exporter go.mod to local dev setup of procfs and comparing the results in both

cc: @discordianfish