thermal: Fix error path in thermal_init()

thermal_unregister_governors() and class_unregister() were being called in
the wrong order.

Fixes: 80a26a5c

 ("Thermal: build thermal governors into thermal_sys module")
Cc: stable@vger.kernel.org
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>