[media] cx25840-core: get rid of warning: no previous prototype

drivers/media/i2c/cx25840/cx25840-core.c:2068:6: warning: no previous prototype for 'cx23885_dif_setup' [-Wmissing-prototypes]

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