Sonoff TRVZB: Fix rounding of externalTemperatureInput

[marazmarci]

As described in Koenkk/zigbee2mqtt#26846, the converter for the Sonoff TRVZB's externalTemperatureInput attribute is prone to
floating-point precision errors. A simple Math.round call is enough to solve this, and specifying precision: 1 will do just that under the hood.

After the fix, it's now possible to set the value to 19.4 and it won't change to 19.3.

[Koenkk]

Thanks!