mfd: lm3533: Switch to using the new API kobj_to_dev()

Fixes the following coccicheck warning:

  drivers/mfd/lm3533-core.c:361:60-61: WARNING opportunity for kobj_to_dev()

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>