net/mlx4_core: Return proper error code when __mlx4_add_one fails

Returning 0 (success) when in fact we are aborting the load, leads to kernel
panic when unloading the module. Fix that by returning the actual error code.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>