[media] exynos-gsc: avoid build warning without CONFIG_OF

When building the exynos-gsc driver with CONFIG_OF disabled, we get
a warning about an out-of-bounds access:

drivers/media/platform/exynos-gsc/gsc-core.c: In function 'gsc_probe':
drivers/media/platform/exynos-gsc/gsc-core.c:1078:34: error: array subscript is above array bounds [-Werror=array-bounds]

This is harmless because the driver will never be used without CONFIG_OF,
but it's better to avoid the warning anyway. Checking the return value
of of_alias_get_id() for an error condition is probably a good idea
anyway, and it makes sure the compiler can verify that we don't get
into that situation.

Fixes: 26a7ed9c ("[media] exynos-gsc: remove an always false condition")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>