hwmon: (pmbus/ltc2978) Fix temperature reporting

commit 8c958c703ef8804093437959221951eaf0e1e664 upstream.

On LTC2978, only READ_TEMPERATURE is supported. It reports
the internal junction temperature. This register is unpaged.

On LTC3880, READ_TEMPERATURE and READ_TEMPERATURE2 are supported.
READ_TEMPERATURE is paged and reports external temperatures.
READ_TEMPERATURE2 is unpaged and reports the internal junction
temperature.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>