hwmon: (it87) Fix VID reading on IT8718F/IT8720F

Comparing apples to bananas doesn't seem right. Consistently use the
chips enum for chip type comparisons, to avoid such bugs in the
future.

The bug has been there since support for the IT8718F was added, so
VID never worked for this chip nor for the similar IT8720F.

Signed-off-by: Jean Delvare <khali@linux-fr.org>