[media] s5p-mfc: Add missing static storage class in s5p_mfc_enc.c file

Fixes the following sparse warnings:
drivers/media/video/s5p-mfc/s5p_mfc_enc.c:1439:5: warning: symbol 'vidioc_s_parm' was not declared. Should it be static?
drivers/media/video/s5p-mfc/s5p_mfc_enc.c:1455:5: warning: symbol 'vidioc_g_parm' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>