hwmon/f71882fg: Set platform drvdata to NULL later

This avoids a possible race leading to trying to dereference NULL.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>