sensor: tmp007: fix negative sample values bug

Fix bug that caused negative temperature values to be computed incorrectly.
The sign bit needs to be perserved, when extracting the data sample from
the TMP007_REG_TOBJ register.

Jira: ZEP-499
Change-Id: Idbebd48b3164d7d4883352ef7128ef73be70e1b2
Signed-off-by: Bogdan Davidoaia <bogdan.m.davidoaia@intel.com>