usb: xhci: remove the use of xhci->addr_dev

xhci->addr_dev is used for the completion of both address device
and enable slot commands. It's shared by enumerations of all USB
devices connected to an xhci host. Hence, it's just a source for
possible races. Since we've introduced command structure and the
command queue to xhci driver.  It is time to get rid of addr_dev
and use the completion in the command structure instead.

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>