RDMA/mlx5: Remove kernel-doc warning in umr.c

Remove extra asterisk to avoid the following kernel-doc warning:

  Warning: drivers/infiniband/hw/mlx5/umr.c:986 This comment starts with '/**',
  but isn't a kernel-doc comment. Refer to Documentation/doc-guide/kernel-doc.rst

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>