hwmon: (ltc2978) Add missing chip IDs for LTC2978 and LTC3882

Add additional chip ID for an older revision of LTC2978, as well
as two chip IDs for LTC3882. Turns out the LTC3882 does support the
LTC2978_MFR_SPECIAL_ID register, and reading it returns its chip ID,
but the register is undocumented.

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