hwmon: (it87) Fix fan tachometer reading in IT8712F rev 0x7 (I)

The IT8712F v0.9.1 datasheet applies to revisions >= 0x8 (J).
The driver was incorrectly attempting to enable 16-bit fan
readings on rev 0x7 (I) which led to incorrect RPM values.

Signed-off-by: Andrew Paprocki <andrew@ishiboo.com>
Tested-by: John Gumb <john.gumb@tandberg.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>