[media] em28xx: fix capture type setting in em28xx_urb_data_copy_vbi()

Set capture type to 1 (video start) when the video frame start header is
detected. This bug didn't cause any trouble, because this type of header is
never received in vbi mode.
Fix it, because we want to use this function with disabled vbi in the future.
Also start with capture type -1 to avoid processing of corrupted/incomplete
frame data which is usually received at streaming start (especially when
USB bulk transfers are used).

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>