IB/mlx5: Fix memory leak in mlx5_ib_create_flow

In case memory resources for *ucmd* were allocated, release them
before return.

Addresses-Coverity-ID: 1469857 ("Resource leak")
Fixes: 3b3233fb

 ("IB/mlx5: Add flow counters binding support")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>