caif_virtio: fix wrong pointer check in cfv_probe()

In the Linux kernel, the following vulnerability has been resolved:

caif_virtio: fix wrong pointer check in cfv_probe()

del_vqs() frees virtqueues, therefore cfv->vq_tx pointer should be checked

for NULL before calling it, not cfv->vdev. Also the current implementation

is redundant because the pointer cfv->vdev is dereferenced before it is

checked for NULL.

Fix this by checking cfv->vq_tx for NULL instead of cfv->vdev before

calling del_vqs().