mlxsw: Update transceiver_overheat counter according to MTWE

MTWE (Management Temperature Warning Event) is triggered when module's
temperature is higher than its threshold.

Register for MTWE events and increase the module's overheat counter when
its corresponding sensor goes above the configured threshold.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>