[media] exynos4-is: Annotate unused functions

__is_set_init_isp_aa and fimc_is_hw_set_tune currently do not have
any callers. However these functions may be used in the future. Hence
instead of deleting them, staticize and annotate them with __maybe_unused
flag to avoid compiler warnings.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>