vdpasim: remove unused variable 'ret'

drivers/vdpa/vdpa_sim/vdpa_sim.c:92:6: warning:
 variable ‘ret’ set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20200410115422.42308-1-yuehaibing@huawei.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>