hwmon: (it87) Fix pwm sysfs attribute removal

Detection if a pwm channel is supported was wrong on removal,
causing the code to try removing non-existing sysfs attributes.
That didn't matter much because sysfs attribute removal of non-existing
files fails silently, and because the wrong evaluation always returned
false, but should nevertheless be fixed.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>