[media] videobuf2: fix an error message

buf->size is not yet initialised in videobuf2-dma-contig at the time of
the error message, use "size."

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>