of: unittest: fix an error test in of_unittest_overlay_8()

We changed this from of_overlay_apply() to overlay_data_apply().  The
overlay_data_apply() function returns 1 on success and 0 on error so
the check for less than zero needs to be updated.

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