sensor: tmp108: fix one-shot mode for as6212

When configuring the device to run in one-shot mode, the AS6212 requires
the sleep mode bit to be set. From the datasheet:

```
The sleep mode is activated by setting the bit SM in the configuration
register to 1. This shuts the device down immediately and reduces the
power consumption to a minimum value.

Entering the sleep mode will take some time (120 ms maximum) and the
first conversion after the sleep mode has been entered takes longer than
the values specified in Figure 7. It is therefore recommended when
entering sleep mode to trigger a single shot conversion at the same
time. After 150 ms (max), the device has then entered the sleep mode and
subsequent conversion times are as specified in Figure 7.
```

Signed-off-by: Jordan Yates <jordan@embeint.com>