[media] exynos4-is: Convert index variable to signed

index variable is used to check the validity of the data by
testing for negative values. Hence make it signed.

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