hwmon: (f71882fg) Cleanup f8000 pwm handling

Currently we are using separate per model sysfs attr for the 3th pwm, because
the 3th pwm of the f8000 only has automatic mode and not manual mode. Doing
things this way was getting in the way for adding f71858fg support, so this
patch makes the pwm attr identical for all models, and instead adds a check
to store_pwm_enable() disallowing setting the 3th pwm to manual mode
on a f8000 IC.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>