[media] em28xx: rename function em28xx_isoc_copy and extend for USB bulk transfers

The URB data processing for 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.

[mchehab@redhat.com: Fix a CodingStyle issue: don't break strings
 into separate lines]

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