hwmon: (pc87427) Handle disabled fan inputs properly

Most fan input pins of the PC87427 can have alternate functions.
Update the driver to check the configuration register and only support
fan inputs which are really used for fan monitoring.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>