[media] s5p-mfc: Optimize copy time stamp handling

For the sake of simplicity and readability memcpy was replaced with
assignment.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>