hwmon: (pmbus/ltc2978) Use detected chip ID to select supported functionality

We read the chip ID from the chip, use it to determine if the chip ID provided
to the driver is correct, and report it if wrong. We should also use the
correct chip ID to select supported functionality.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # 3.2+
Acked-by: Jean Delvare <khali@linux-fr.org>