mlxsw: core: Add API for QSFP module temperature thresholds reading

Add new API to read QSFP module's temperature thresholds - warning and
critical.

New internal API reads the temperature thresholds from the modules,
which are equipped with the thermal sensor. These thresholds will be
exposed via hwmon subsystem.

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