mlxsw: spectrum_ethtool: Expose transceiver_overheat counter

Add structures for port statistics which read from core and not directly
from registers.

When netdev's ethtool statistics are queried, query the corresponding
module's overheat counter from core and expose it as
"transceiver_overheat".

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