[media] s5p-mfc: Fix compilation warning

Added missing const qualifier.
Without this patch compiler gives the following warning:
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c:1576:2: warning:
initialization from incompatible pointer type [enabled by default]
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c:1576:2: warning:
(near initialization for ‘s5p_mfc_enc_ioctl_ops.vidioc_subscribe_event’)
[enabled by default]

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>