USB: xhci: Don't wait for a disable slot cmd when HC dies.

When the host controller dies or is removed while a device is plugged in,
the USB core will attempt to deallocate the struct usb_device.  That will
call into xhci_free_dev().  This function used to attempt to submit a
disable slot command to the host controller and clean up the device
structures when that command returned.  Change xhci_free_dev() to skip the
command submission and just free the memory if the host controller died.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>