hwmon: (ads7871) Add 'name' sysfs attribute

The 'name' sysfs attribute is mandatory for hwmon devices, but was missing
in this driver.

Cc: Paul Thomas <pthomas8589@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Paul Thomas <pthomas8589@gmail.com>