hw/isa/piix3: Resolve redundant k->config_write assignments

hw/isa/piix3: Resolve redundant k->config_write assignments

The previous patch unified handling of piix3_write_config() accross the
PIIX3 device models which allows for assigning k->config_write once in the
base class.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Chuck Zmudzinski <brchuckz@aol.com>
Message-Id: <20230312120221.99183-6-shentey@gmail.com>
Message-Id: <20230403074124.3925-7-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>