[media] mipi-csis: get rid of a warning

drivers/media/platform/exynos4-is/mipi-csis.c: In function 's5pcsis_parse_dt':
drivers/media/platform/exynos4-is/mipi-csis.c:756:2: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  if (state->index < 0 || state->index >= CSIS_MAX_ENTITIES)
  ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>