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: NShuah Khan <shuahkh@osg.samsung.com>
Acked-by: NLad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>