[media] em28xx: use common urb data copying function for vbi and non-vbi data streams

em28xx_urb_data_copy_vbi() is actually an extended version of
em28xx_urb_data_copy(). With the preceding fixes and improvements, it works
fine with both, vbi and non-vbi data streams without performance impacts.
So rename em28xx_urb_data_copy_vbi() to em28xx_urb_data_copy(), delete the
the old implementation of em28xx_urb_data_copy() and change the code to use
this function for both data stream types.
Tested with "SilverCrest 1.3 MPix webcam" (progressive, non-vbi) and
"Hauppauge HVR-900 (65008/A1C0)" (interlaced, vbi enabled and disabled).

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