V4L/DVB (12746): em28xx: do not create /dev/vbiX device if VBI not supported

Do not create the VBI device in cases where VBI is not supported on the target
em28xx chip.

This work was sponsored by EyeMagnet Limited.

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