V4L/DVB (12741): em28xx: make video isoc stream work when VBI is enabled

Add code enabling the VBI registers for variants of the em28xx chip that
support VBI, and make sure the isoc streaming code continues to work for
the video component of the stream (note the video and vbi data arrive
intermixed on the same isoc pipe).

Note that this version just drops the actual VBI data onto the floor as
opposed to processing it.  The "#ifdef 0" tags are for the videobuf code that
appears in the next patch in this series.

We created a separate version of the isoc_copy version for parsing the version
of the stream that includes VBI data.  In theory, they might be able to be
merged at some point in the future, but the initial goal is to ensure that we
do not cause any regressions with devices that do not have VBI support.

This work was sponsored by EyeMagnet Limited.

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