V4L/DVB: v4l: mem2mem_testdev: fix errorenous comparison

Output buffer has to be at least the size of input buffer, not the other
way around.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>