V4L/DVB (3421): Several fixes to prepare for VBI

- Fixed VBI compilation.
- Included capacity to specify vbi and video number.
- Added a better control for using more than one em28xx device.
- VIDIOC_G_FMT now calls a function.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>