[media] mem2mem_testdev: Add missing braces around sizeof

Fixes the following checkpatch warnings:
WARNING: sizeof *ctx should be sizeof(*ctx)
WARNING: sizeof *dev should be sizeof(*dev)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>