drivers: sensor: tmp108: fix coverity integer handling issue

Fix coverity integer handling issue (CWE-188).
Modifying a variable through a pointer of an incompatible type (other
than unsigned char) can lead to unpredictable results.

Fix: #67965
Coverity-CID: 248434

Signed-off-by: Armando Visconti <armando.visconti@st.com>