mlxsw: core_thermal: Simplify transceiver module get_temp() callback

The get_temp() callback of a thermal zone associated with a transceiver
module no longer needs to read the temperature thresholds of the module.
Therefore, simplify the callback by only reading the temperature.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>