Comment 8 for bug 1882123

Hi all,

I'm sure this patch will prevent the assertion failure due to the
inconsistent ep and pid (UBS_TOKEN_SETUP) (
https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg07179.html).

For UHCI (https://gitlab.com/qemu-project/qemu/-/issues/119) and OHCI (
https://gitlab.com/qemu-project/qemu/-/issues/303), this patch may be
right.

For EHCI, I found another way to trigger this assertion even with my patch
because ehci_get_pid() returns 0 if qtd->token.QTD_TOKEN_PID is not
valid[0]. In this case, the patch cannot capture it because pid is zero[2].
This case is specific to EHCI as far as I know. It seems we want to drop
the operation if ehci_get_pid() returns 0.

```static int ehci_get_pid(EHCIqtd *qtd)
{
    switch (get_field(qtd->token, QTD_TOKEN_PID)) {
    case 0:
        return USB_TOKEN_OUT;
    case 1:
        return USB_TOKEN_IN;
    case 2:
        return USB_TOKEN_SETUP;
    default:
        fprintf(stderr, "bad token\n"); //
---------------------------------------------> [0]
        return 0;
    }
}

static int ehci_execute(EHCIPacket *p, const char *action)
{
    p->pid = ehci_get_pid(&p->qtd); //
--------------------------------------------> [1]
    p->queue->last_pid = p->pid;
    endp = get_field(p->queue->qh.epchar, QH_EPCHAR_EP);
    ep = usb_ep_get(p->queue->dev, p->pid/*=0*/, endp); //
-----------------------> [2]
```

A qtest sequence is like
```
writel 0x1011b000 0x10124000
writel 0x10124004 0x358cbd80
writel 0x10124018 0x9e4bba36
writel 0x10124014 0x10139000
writel 0xfebd5020 0x1c4a5135
writel 0x10139008 0x3d5c4b84
clock_step 0xb17b0
writel 0xfebd5064 0x5f919911
clock_step 0xa9229
writel 0xfebd5064 0x5431e207
writel 0xfebd5038 0x1b2034b5
writel 0x1b2034a0 0x10100000
writel 0x10100000 0x10109000
writel 0x10109000 0x1011b000
clock_step 0xa9229
```

Best,
Qiang