[media] media: fix au0828 compile error from au0828_boards initialization

au0828 picked up UNSET from videobuf-core.h and fails to compile
if videobuf-core.h isn't included. Change it to use -1U instead
to fix the problem.

    drivers/media/usb/au0828/au0828-cards.c:47:17: error: ‘UNSET’ undeclared here (not in a function)
       .tuner_type = UNSET,

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>