[media] em28xx: rename module parameter prefer_bulk to usb_xfer_mode

Since we have now 3 modes (auto/isoc/bulk), usb_xfer_mode is more suitable than prefer_bulk.

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