[media] em28xx: fix video data start position calculation in em28xx_urb_data_copy_vbi()

The header check/removal code at the end of function em28xx_urb_data_copy_vbi()
is obsolete, because this is already done earlier in this function.
In fact it is incomplete (doesn't check for vbi header) and causes trouble
when the first data bytes are the same as header bytes (which is fortunately
very unlikely).

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