Both ngene and ddbrige calls dvb_attach once for drxk_attach.
The logic used there, and by tda18271c2dd driver is different
from similar logic on other frontends.

The right fix is to change them to use the same logic, but,
while we don't do that, we need to patch em28xx-dvb in order
to do cope with ngene/ddbridge magic.

While here, document why drxk_t_release should do nothing.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>