[media] s5p-mfc: Use WARN_ON(condition) directly

Use WARN_ON(condition) directly instead of wrapping around an if
condition.

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>