md/raid5: missing error code in setup_conf()

Return -ENOMEM if the allocation fails.  Don't return success.

Fixes: 8fbcba6b ("md/raid5: Cleanup setup_conf() error returns")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Song Liu <song@kernel.org>