[media] s5k6aa: Add missing static storage class specifier

Fixes the following sparse warning:
drivers/media/video/s5k6aa.c:1439:5: warning:
symbol 's5k6aa_check_fw_revision' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>