mlxsw: core: Add support for negative temperature readout

Extend macros MLXSW_REG_MTMP_TEMP_TO_MC() to allow support of negative
temperature readout, since chip and others thermal components are
capable of operating within the negative temperature.
With no such support negative temperature will be consider as very high
temperature and it will cause wrong readout and thermal shutdown.
For negative values 2`s complement is used.
Tested in chamber.
Example of chip ambient temperature readout with chamber temperature:
-10 Celsius:
temp1:             -6.0C  (highest =  -5.0C)
-5 Celsius:
temp1:             -1.0C  (highest =  -1.0C)

v2 (Andrew Lunn):
* Replace '%u' with '%d' in mlxsw_hwmon_module_temp_show()

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>