mlxsw: core_hwmon: Calculate MLXSW_HWMON_ATTR_COUNT more accurately

Currently the value of MLXSW_HWMON_ATTR_COUNT is calculated not really
accurate.

Add several defines to make the calculation clearer and easier to
change.

Calculate the precise high bound of number of attributes that may be
needed.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>