power: supply: cpcap-charger: Fix enable for 3.8V charge setting

Zero is a valid register value for for 3.8V charging.

Fixes: 0c9888e3 ("power: supply: cpcap-charger: Add minimal
CPCAP PMIC battery charger")
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>