xhci: zero usb device slot_id member when disabling and freeing a xhci slot

set udev->slot_id to zero when disabling and freeing the xhci slot.
Prevents usb core from calling xhci with a stale slot id.

xHC controller may be reset during resume to recover from some error.
All slots are unusable as they are disabled and freed.
xhci driver starts slot enumeration again from 1 in the order they are
enabled. In the worst case a stale udev->slot_id for one device matches
a newly enabled slot_id for a different device, causing us to
perform a action on the wrong device.

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