Make sure videobuf_waiton() is used before freeing a buffer.
Without this fix we may return the buffer to the allocator
before the bus mastering operation is finished.

Reported-by: Matthieu CASTET <matthieu.castet@parrot.com>
Tested-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>