hwmon: (it87) Fix pwm4 detection for IT8620 and IT8628

pwm4 is enabled if bit 2 of GPIO control register 4 is disabled,
not when it is enabled. Since the check is for the skip condition,
it is reversed. This applies to both IT8620 and IT8628.

Fixes: 36c4d98a ("hwmon: (it87) Add support for all pwm channels ...")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>