power: supply: cpcap-battery: Fix low battery check

We need to check current instead of the charge counter to see if
a charger is connected. The charge counter shows the cumulated value
instead of the current charge current and can be negative or positive.

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