hwmon: (ltc2978) Use correct ID mask to detect all chips

Per information from Linear Technologies, the ID mask is 12 bit
for all chips of this series. Use this mask to detect chips to ensure
that all chip revisions are detected.

Suggested-by: Michael Jones <mike@proclivis.com>
Tested-by: Michael Jones <mike@proclivis.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>