[media] s5p-mfc: declare s5p_mfc_bus_reset as static

drivers/media/platform/s5p-mfc/s5p_mfc_ctrl.c:132:5: warning: no previous prototype for 's5p_mfc_bus_reset' [-Wmissing-prototypes]
 int s5p_mfc_bus_reset(struct s5p_mfc_dev *dev)
     ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>