vDPA/ifcvf: Fix return value check for vdpa_alloc_device()

The vdpa_alloc_device() returns an error pointer upon
failure, not NULL. To handle the failure correctly, this
replaces NULL check with IS_ERR() check and propagate the
error upwards.

Fixes: 5a2414bc

 ("virtio: Intel IFC VF driver for VDPA")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-3-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>