hwmon: (f71882fg) Sanity check f8000 pwm settings

We depend up on the pwm_enable register (0x96) not containing any reserved
settings in various places. We were already checking to make sure there
were no reserved settings in the register for the f71862fg, this patch adds
the same checking for the f8000, while at it it also moves the code to
a more apropriate place so we don't need to check if the fan/pwm part
of the IC is enabled twice.

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