[media] s5p-mfc: Fix encoder control 15 issue

mfc-encoder is not working in the latest kernel giving the
erorr "Adding control (15) failed". Adding the missing step
parameter in this control to fix the issue.

Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>