power: supply: cpcap-battery: Fix signed counter sample register

The accumulator sample register is signed 32-bits wide register on
droid 4. And only the earlier version of cpcap has a signed 24-bits
wide register. We're currently passing it around as unsigned, so
let's fix that and use sign_extend32() for the earlier revision.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>