hwmon: (dell-smm-hwmon) Use a valid name attribute

As per Documentation/hwmon/sysfs-interface, hwmon name attributes must
not include '-', so replace 'dell-smm' with 'dell_smm'.

Fixes: 039ae585 ("hwmon: Allow to compile dell-smm-hwmon driver without /proc/i8k")
Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>