[media] em28xx: ignore isoc DVB USB endpoints with wMaxPacketSize = 0 bytes for all alt settings

Some devices without DVB support (such as the "Terratec Grabby" and
"Easycap DC-60") provide isochronous DVB USB endpoints with wMaxPacketSize set
to 0 bytes for all alt settings.
Ignore these endpoints and avoid registering a DVB device node and loading the
DVB driver extension.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Tested-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>