USB: xhci: Avoid double free after streams are disabled.

USB: xhci: Avoid double free after streams are disabled.

When a device is disconnected, xhci_free_virt_device() is called.  Ramya
found that if the device had streams enabled, and then the driver freed
the streams with a call to usb_free_streams(), then about a minute after
he had called this, his machine crashed with a Bad DMA error.  It turns
out that xhci_free_virt_device() would attempt to free the endpoint's
stream_info data structure if it wasn't NULL, and the free streams
function was not setting it to NULL after freeing it.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Ramya Desai <ramya.desai@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>