net/mlx4_core: Fix smatch error - possible access to a null variable

Fix the "error: we previously assumed 'out_param' could be null" found
by smatch semantic checker on:
drivers/net/ethernet/mellanox/mlx4/cmd.c:506 mlx4_cmd_poll()
drivers/net/ethernet/mellanox/mlx4/cmd.c:578 mlx4_cmd_wait()

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>