usb: xhci: fix spinlock recursion for USB2 test mode

Both xhci_hub_control and xhci_disable_slot tries to hold spinlock, the
spinlock recursion occurs when enters USB2 test mode. Fix it by unlock
spinlock before calling xhci_disable_slot.

Cc: <stable@vger.kernel.org>
Fixes: 0f1d832e ("usb: xhci: Add port test modes support for usb2")
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>