usb: xhci: cleanup cmd_completion in xhci_virt_device

cmd_completion in struct xhci_virt_device is legacy. With command
structure and command queue introduced in xhci, cmd_completion is
not used any more. This patch removes it.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>