drivers: sensor: explorir_m: improve transceive

During calibration feature development the following loop was used for
testing:

```shell
CMD="sensor attr_set explorir_m co2 calibration 1234"
while : ; do echo "$CMD" > /dev/ttyACM0; sleep .8; done
```

At some point calibration would fail and all following calibration commands
would fail.

Reset buffer and semaphore just before terminating the command to better
mitigate disturbance from previous failed command.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>