[media] s5p-mfc: Set last buffer flag

Setting the last buffer flag causes the videobuf2 core to return -EPIPE from
DQBUF calls on the capture queue after the last buffer is dequeued.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>