hwmon: (it87) Verify the VID pin usage

The VID input pins can alternatively be used as GPIOs. Make sure we
have at least 4 pins used for VID, otherwise don't bother reading and
exposing VID.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Adam Nielsen <a.nielsen@shikadi.net>