Extend all cable info APIs with 'slot_index' argument.

For main board, slot will always be set to zero and these APIs will work
as before. If reading cable information is required from cages located
on line cards, slot should be set to the physical slot number, where
line card is located in modular systems.
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>