hwmon: Reduce indentation level in __hwmon_device_register()

Reduce indentation level in __hwmon_device_register() by preparing a
helper function.

This just improves code readability.  No functional change.

Cc: Jean Delvare <jdelvare@suse.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>