vfio: platform: fix reference leak in vfio_platform_open

pm_runtime_get_sync() will increment pm usage counter even it
failed. Forgetting to call pm_runtime_put will result in
reference leak in vfio_platform_open, so we should fix it.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Acked-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>