mlxsw: minimal: Extend APIs with slot index for modular system support

Add 'slot_index' field to port structure.
Replace zero slot_index argument with 'slot_index' in 'ethtool'
related APIs.
Add 'slot_index' argument to port initialization and
de-initialization related APIs.

Motivation is to prepare minimal driver for modular system support.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>