mlxsw: core: Fix temperature sensor index during initialization

Sensor index should be passed instead of 0. For now, this does not make
a difference, since there is so far only one temperature sensor
exposed by HW.

Fixes: 89309da3 ("mlxsw: core: Implement temperature hwmon interface")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>