of/unittest: Swap arguments of of_unittest_apply_overlay()

Function signature

  of_unittest_apply_overlay(int unittest_nr, int overlay_nr, ...

and call sites, like in of_unittest_apply_overlay_check():

  ret = of_unittest_apply_overlay(overlay_nr, unittest_nr, ...

do not match. Fix this in one place (function signature).
The only affected test case is 15, which supplies non-existing
overlay number 16, but two bugs matched here. Fix the test case.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>