hwmon: (w83627ehf) Move fan pins check to a separate function

Move the check of fan pin usage to a separate function. This improves
readability, and will make it easier to integrate chip-specific
conditions.

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