[media] s5p-mfc: Add missing braces around sizeof

Silences the following 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: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>