xhci: rename xhci_complete_packet to xhci_try_complete_packet

Make clear that this isn't guaranteed to actually complete the transfer,
the usb packet can still be in flight after calling that function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1485790607-31399-3-git-send-email-kraxel@redhat.com