media: davinci/vpbe: array underflow in vpbe_enum_outputs()

In vpbe_enum_outputs() we check if (temp_index >= cfg->num_outputs) but
the problem is that "temp_index" can be negative.  This patch changes
the types to unsigned to address this array underflow bug.

Fixes: 66715cdc ("[media] davinci vpbe: VPBE display driver")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: "Lad, Prabhakar" <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>