[media] exynos-gsc: Add missing video device vfl_dir flag initialization

vfl_dir should be set to VFL_DIR_M2M so valid ioctls for this
mem-to-mem device can be properly determined in the v4l2 core.

Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>