IB/mlx5: Fix return value check in flow_counters_set_data()

In case of error, the function mlx5_fc_create() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Fixes: 3b3233fb

 ("IB/mlx5: Add flow counters binding support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>