power: supply: cpcap-battery: fix invalid usage of list cursor

Fix invalid usage of a list_for_each_entry in cpcap_battery_irq_thread().
Empty list or fully traversed list points to list head, which is not
NULL (and before the first element containing real data).

Signed-off-by: Guangqing Zhu <zhuguangqing83@gmail.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Carl Philipp Klemm <philipp@uvos.xyz>
Tested-by: Carl Philipp Klemm <philipp@uvos.xyz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>