usb: Set USBEndpoint in usb_packet_setup().

With the separation of the device lookup (via usb_find_device) and
packet processing we can lookup device and endpoint before setting up
the usb packet.  So we can initialize USBPacket->ep early and keep it
valid for the whole lifecycle of the USBPacket.  Also the devaddr and
devep fields are not needed any more.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
