[media] em28xx: em28xx_urb_data_copy(): move duplicate code for capture_type=0 and capture_type=2 to a function

Reduce code duplication by moving the duplicate code for dev->capture_type=0
(vbi start) and dev->capture_type=2 (video start) to a function.
The same function will also be called by the (not yet existing) em25xx frame
data processing code.
Signed-off-by: NFrank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>