Fix the following gcc warning:

drivers/power/supply/max14656_charger_detector.c:142:6: warning:
variable ‘ret’ set but not used [-Wunused-but-set-variable]
  int ret = 0;
      ^~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>