usb: xhci: Add helper function xhci_disable_slot().

Refactoring slot disable related code into a helper
function xhci_disable_slot() which can be used when
enabling test mode.

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