This patch fixes the following bug:
The thermal framework creates a temp*_crit interface, only when
the get_crit_temp pointer is defined. But during the _unregister_
path, without checking for the function definition, it is trying to
remove the temp*_crit interface.

Signed-off-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>