mlxsw: core_hwmon: Introduce slot parameter in hwmon interfaces

Add 'slot' parameter to 'mlxsw_hwmon_dev' structure. Use this parameter
in mlxsw_reg_mtmp_pack(), mlxsw_reg_mtbr_pack(), mlxsw_reg_mgpir_pack()
and mlxsw_reg_mtmp_slot_index_set() routines.
For main board it'll always be zero, for line cards it'll be set to
the physical slot number in modular systems.

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: David S. Miller <davem@davemloft.net>