thermal: Add comment explaining test for critical temperature

The code testing if a temperature should be emulated or not is
not obvious. Add a comment explaining why this test is done.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>