usb:xhci:fix memory leak in usb_xhci_exit

If the xhci uses msix, it doesn't free the corresponding
memory, thus leading a memory leak. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-id: 57d7d2e0.d4301c0a.d13e9.9a55@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
