gpio: virtio: Fix sparse warnings

Fix warnings reported by sparse, related to type mismatch between u16
and __le16.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 3a29355a ("gpio: Add virtio-gpio driver")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>