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

commit f366fccd0809f13ba20d64cae3c83f7338c88af7 upstream.

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>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>