V4L/DVB (11077): au0828: properly handle missing analog USB endpoint

Move the setup of the analog isoc handler into au0828-video.c, so it does not
occur if there is not an .input section defined for the board.  Also fixes a
case where if there is an input section but the board does not actually have
analog support, the digital support will continue to work as expected.

Thanks to Michael Krufky <mkrufky@linuxtv.org> for providing sample hardware
of various configurations to test with.

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>