RDMA/mlx5: Fix type warning of sizeof in __mlx5_ib_alloc_counters()

sizeof() when applied to a pointer typed expression should give the size
of the pointed data, even if the data is a pointer.

Fixes: e1f24a79

 ("IB/mlx5: Support congestion related counters")
Link: https://lore.kernel.org/r/20200917081354.2083293-1-liushixin2@huawei.com
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>