IB/mlx5: Fix error code in get_port_caps()

The current code returns success when kmalloc() fails.  It should
return an error code, -ENOMEM.

Fixes: e126ba97

 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>