power: supply: test_power: add missing newlines when printing parameters by sysfs

When I cat some module parameters by sysfs, it displays as follows.
It's better to add a newline for easy reading.

root@syzkaller:~# cd /sys/module/test_power/parameters/
root@syzkaller:/sys/module/test_power/parameters# cat ac_online
onroot@syzkaller:/sys/module/test_power/parameters# cat battery_present
trueroot@syzkaller:/sys/module/test_power/parameters# cat battery_health
goodroot@syzkaller:/sys/module/test_power/parameters# cat battery_status
dischargingroot@syzkaller:/sys/module/test_power/parameters# cat battery_technology
LIONroot@syzkaller:/sys/module/test_power/parameters# cat usb_online
onroot@syzkaller:/sys/module/test_power/parameters#

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>