[media] dm355_ccdc: declare a function as static

drivers/media/platform/davinci/dm355_ccdc.c:463:5: warning: no previous prototy
pe for 'ccdc_write_dfc_entry' [-Wmissing-prototypes]
 int ccdc_write_dfc_entry(int index, struct ccdc_vertical_dft *dfc)
     ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>