PM: runtime: Fix error checking for kunit_device_register()

The kunit_device_register() function never returns NULL, it returns
error pointers.  Update the assertions to use
KUNIT_ASSERT_NOT_ERR_OR_NULL() instead of checking for NULL.

Fixes: 7f7acd19 ("PM: runtime: Add basic kunit tests for API contracts")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>