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 <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: Benjamin Gaignard <[e-mail]>
Cc: Brian Starkey <[e-mail]>
Cc: Laura Abbott <[e-mail]>
Cc: "Andrew F. Davis" <[e-mail]>
Cc: [e-mail]
Signed-off-by: John Stultz <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/dmabuf-heaps/dmabuf-heap.c

···
 	}
 
 	printf("Expected error checking passed\n");
+	ret = 0;
 out:
 	if (dmabuf_fd >= 0)
 		close(dmabuf_fd);