mlxsw: core_thermal: Use common define for thermal zone name length

Replace internal define 'MLXSW_THERMAL_ZONE_MAX_NAME' by common
'THERMAL_NAME_LENGTH'.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>