RDMA/mlx5: Remove impossible check of mkey cache cleanup failure

mlx5_mkey_cache_cleanup() can't fail and can be changed to be void.

Link: https://lore.kernel.org/r/1acd9528995d083114e7dec2a2afc59436406583.1675328463.git.leon@kernel.org


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