V4L/DVB (12747): em28xx: only advertise VBI capability if supported

Change the code so we only claim to support VBI if the underlying chipset
actually has the 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>