[media] em28xx: rename function em28xx_dvb_isoc_copy and extend for USB bulk transfers

The URB data processing for DVB bulk transfers is very similar to
what is done with isoc transfers, so create a common function that
works with both transfer types based on the existing isoc function.
Tested with device Hauppauge HVR-930c.

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