V4L/DVB: em28xx: reduce cropping for VBI area

It turns up we can reduce the starting line for the active area, which results
in more data being captured when under PAL (while the full VBI capture window
still stays properly encoded).

This work was sponsored by EyeMagnet Limited.

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