[media] exynos-gsc: Fix compilation warning

Used type casting to avoid the following compilation warning:
drivers/media/platform/exynos-gsc/gsc-core.c:983:37: warning:
incorrect type in assignment (different modifiers)
drivers/media/platform/exynos-gsc/gsc-core.c:983:37:
expected struct gsc_driverdata *driver_data
drivers/media/platform/exynos-gsc/gsc-core.c:983:37:
got void const *data

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>