hwmon: (ab8500) Call kernel_power_off instead of pm_power_off

Drivers should not call pm_power_off directly; it is not guaranteed
to be non-NULL. Call kernel_power_off instead.

Cc: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>