[media] pwc: better usb disconnect handling

Unplugging a pwc cam while an app has the /dev/video# node open leads
to an oops in pwc_video_close when the app closes the node, because
the disconnect handler has free-ed the pdev struct pwc_video_close
tries to use. Instead of adding some sort of bandaid for this.
fix it properly using the v4l2 core's new(ish) behavior of keeping the
v4l2_dev structure around until both unregister has been called, and
all file handles referring  to it have been closed:

Embed the v4l2_dev structure in the pdev structure and define a v4l2 dev
release callback releasing the pdev structure (and thus also the embedded
v4l2 dev structure.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/pwc/pwc-ctrl.c

@@ -1414,7 +1414,7 @@ long pwc_ioctl(struct pwc_device *pdev, unsigned int cmd, void *arg)
 	{
 		ARG_DEF(struct pwc_probe, probe)
 
-		strcpy(ARGR(probe).name, pdev->vdev->name);
+		strcpy(ARGR(probe).name, pdev->vdev.name);
 		ARGR(probe).type = pdev->type;
 		ARG_OUT(probe)
 		break;

drivers/media/video/pwc/pwc.h

@@ -162,9 +162,9 @@ struct pwc_imgbuf
 
 struct pwc_device
 {
-   struct video_device *vdev;
+	struct video_device vdev;
 
-   /* Pointer to our usb_device */
+   /* Pointer to our usb_device, may be NULL after unplug */
    struct usb_device *udev;
 
    int type;                    /* type of cam (645, 646, 675, 680, 690, 720, 730, 740, 750) */