virtio_gpio: drop packed attribute

Declaring the struct packed here is mostly harmless,
but gives a bad example for people to copy.
As the struct is packed and aligned manually,
let's just drop the attribute.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>