of: unittest: fix an error code in of_unittest_apply_overlay()

We accidentally return zero on failure instead of a negative error code.

Fixes: 39a751a4 ("of: change overlay apply input data from unflattened to FDT")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rob Herring <robh@kernel.org>