ehci: Fix memory leak in handling of NAK-ed packets

Currently each time we try to execute a NAK-ed packet we redo
ehci_init_transfer, and usb_packet_map, re-allocing (without freeing) the
sg list every time.

This patch fixes this, it does this by introducing another async state, so
that we also properly cleanup a NAK-ed packet on cancel.

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