net/mlx4_core: Fix inaccurate return value of mlx4_flow_attach()

Adopt the "info: why not propagate 'ret' from parse_trans_rule()..."
suggestion made by the smatch semantic checker on:
drivers/net/ethernet/mellanox/mlx4/mcg.c:867 mlx4_flow_attach()

Signed-off-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>