power: supply: max17042_battery: Use sign_extend32 instead of DIY code

Use sign_extend32 to sign-extend variables where necessary instead of
DIY code.

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>