[media] exynos4-is: Fix off-by-one valid range checking for is->config_index

Current code uses is->config_index as array subscript, thus the valid value
range is 0 ... ARRAY_SIZE(cmd) - 1.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>