[media] em28xx: refactor VBI data processing code in em28xx_urb_data_copy()

When a new frame header is detected in em28xx_urb_data_copy() and the data
packet contains both, VBI data and video data, the prevoius VBI buffer doesn't
get finished and is overwritten with the new VBI data.
This bug is not triggered with isochronous USB transfers, because the data
packetes are much smaller than the VBI data size.
But when using USB bulk transfers, the whole data of an URB is treated as
single packet, which is usually much larger then the VBI data size.
Refactor the VBI data processing code to fix this bug, but also to simplify the
code and make it similar to the video data processing code part (which allows
further code abstraction/unification in the future).
The changes have been tested with device "Hauppauge HVR-900".

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