IB/mlx5: Fix MR cache initialization

Schedule MR cache work only after bucket was initialized.

Cc: <stable@vger.kernel.org> # 4.10
Fixes: 49780d42

 ("IB/mlx5: Expose MR cache for mlx5_ib")
Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>