[media] s5p-mfc: Add missing static storage class to silence warnings

Fixes the following sparse warnings:

drivers/media/video/s5p-mfc/s5p_mfc.c:73:6
	warning: symbol 's5p_mfc_watchdog' was not declared. Should it be static?
drivers/media/video/s5p-mfc/s5p_mfc_opr.c:299:6:
	warning: symbol 's5p_mfc_set_shared_buffer' 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>