media: v4l2-ioctl.c: fix timestamp format

The timestamp uses microseconds, not nanoseconds, so use %06ld instead
of %09ld.

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