[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>