thermal: core: small style fix when checking for __find_governor()

Remove style issue:
CHECK: Comparison to NULL could be written "!__find_governor"
+	if (__find_governor(governor->name) == NULL) {

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>