[media] em28xx: make ioctls VIDIOC_G/S_PARM working for VBI devices

With the current code V4L2_BUF_TYPE_VIDEO_CAPTURE is accepted only, but for VBI
devices only buffer type V4L2_BUF_TYPE_VBI_CAPTURE is used/valid.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>