hwmon: (nct6683) remove useless function

Fix the following clang warning:

drivers/hwmon/nct6683.c:491:19: warning: unused function 'in_to_reg'
[-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1618293770-55307-1-git-send-email-jiapeng.chong@linux.alibaba.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>