power: supply: cpcap-battery: Simplify short term power average calculation

We can use sign_extend32() here to simplify things. And let's fix the
comment for CCM register, that contains the calibration offset.

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