hwmon: (it87) Enhance validation for fan4 and fan5

Several of the chips supported by this driver have a configuration
register to enable fan4 and fan5. Use those registers to determine
if fan4 and fan5 tachometers are supported.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>