IB/mlx5: Fix a NULL vs IS_ERR() check

The kzalloc() function returns NULL, not error pointers.

Fixes: 30f2fe40 ("IB/mlx5: Introduce UAPIs to manage packet pacing")
Link: https://lore.kernel.org/r/20200320132641.GF95012@mwanda


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>