V4L/DVB: Add closed captioning support for the HVR-950q

Add NTSC closed captioning support for au0828 based products.  Note that this
also required reworking the locking to support streaming on both the video and
VBI devices (the logic for which I copied from my changes made to the em28xx
several months ago).

This work was sponsored by GetWellNetwork Inc.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>