V4L/DVB (13932): em28xx: add PAL support for VBI

Make the VBI support work for PAL standards in addition to NTSC.

This work was sponsored by EyeMagnet Limited.

Thanks go out to Andy Walls for providing a CD containing test PAL/VBI captures
and to Steven Toth for providing a PVR-350 to do signal generation with.

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