kselftests: dmabuf-heaps: Fix confused return value on expected error testing

When I added the expected error testing, I forgot I need to set
the return to zero when we successfully see an error.

Without this change we only end up testing a single heap
before the test quits.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: "Andrew F. Davis" <afd@ti.com>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>