V4L/DVB (12742): em28xx: add raw VBI support for NTSC

Add support for raw VBI capture for the em28xx bridge, currently only for
NTSC.  Support for PAL capture to follow shortly (including the removal of
numerous hard-coded NTSC-specific sizes for capture buffers, etc).

Note that the code currently changes the default current norm from PAL to
NTSC (so that zvbi-ntsc-cc works properly).  The default norm really should
be moved into a board-level parameter.

This work was sponsored by EyeMagnet Limited.

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