xhci: handle port status events for removed USB3 hcd

At xhci removal the USB3 hcd (shared_hcd) is removed before the primary
USB2 hcd. Interrupts for port status changes may still occur for USB3
ports after the shared_hcd is freed, causing  NULL pointer dereference.

Check if xhci->shared_hcd is still valid before handing USB3 port events

Cc: <stable@vger.kernel.org>
Reported-by: Peter Chen <peter.chen@nxp.com>
Tested-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>