media: zoran: use v4l2_buffer_set_timestamp

The ns_to_timeval function is removed, so replace it with
v4l2_buffer_set_timestamp().

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>