usb-ehci: Fix cerr tracking

usb-ehci: Fix cerr tracking

cerr should only be decremented on errors which cause XactErr to be set, and
when that happens the failing transaction should be retried until cerr reaches
0 and only then should USBSTS_ERRINT be set (and inactive cleared and
USBSTS_INT set if requested).

Since we don't have any hardware level errors (and in case of redirection
the real hardware has already retried), re-trying makes no sense, so
immediately set cerr to 0 on errors which set XactErr.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>