RDMA/mlx5: Check for error return in flow_rule rather than err

Currently when the call to create_flow_rule_vport_sq fails, the error
check is being performed on err rather than on the return pointer
flow_rule.  The return flow_rule maybe NULL (which is not considered an
error) or an error code, so check for the error on flow_rule.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: d5ed8ac3 ("RDMA/mlx5: Move default representors SQ steering to rule to modify QP")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>