mlxsw: minimal: Return -ENOMEM on allocation failure

These error paths return success but they should return -ENOMEM.

Fixes: 01328e23

 ("mlxsw: minimal: Extend module to port mapping with slot index")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/YwjgwoJ3M7Kdq9VK@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>