mlx4: fix kfree on error path in new_steering_entry()

On error path kfree() should get pointer to memory allocated by
kmalloc() not the address of variable holding it (which is on stack).

Signed-off-by: Mariusz Kozlowski <mk@lab.zgora.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>