power: supply: max17047_battery: The temp alert values are 8-bit 2's complement

The temp alert values are 8-bit 2's complement, so sign-extend them
before reporting them back to the caller.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>