mlxsw: reg: Extend MCION register with new slot number field

Extend MCION (Management Cable IO and Notifications Register) with new
field specifying the slot number. The purpose of this field is to
support access to MCION register for query cage transceiver on modular
system.

For non-modular systems the 'module' number uniquely identifies the
transceiver location. For modular systems the transceivers are
identified by two indexes:
- 'slot_index', specifying the slot number, where line card is located;
- 'module', specifying cage transceiver within the line card.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>