drm/amdgpu: potential NULL dereference in debugfs code

debugfs_create_file() returns NULL on error, it only returns error
pointers if debugfs isn't enabled in the config and we checked for that
earlier so it can't happen.

Fixes: 4f4824b5 ('drm/amd/amdgpu: Convert ring debugfs entries to binary')
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c

@@ -345,8 +345,8 @@ static int amdgpu_debugfs_ring_init(struct amdgpu_device *adev,
 	ent = debugfs_create_file(name,
 				  S_IFREG | S_IRUGO, root,
 				  ring, &amdgpu_debugfs_ring_fops);
-	if (IS_ERR(ent))
-		return PTR_ERR(ent);
+	if (!ent)
+		return -ENOMEM;
 
 	i_size_write(ent->d_inode, ring->ring_size + 12);
 	ring->ent = ent;