[media] s5p-mfc: Make 'clk_ref' static in s5p_mfc_pm.c

Fixes the following sparse warning:
drivers/media/platform/s5p-mfc/s5p_mfc_pm.c:31:10: warning:
symbol 'clk_ref' was not declared. Should it be static?

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>