[media] exynos4-is: Fix potential NULL pointer dereference

dev->of_node could be NULL. Hence check for the same and return before
dereferencing it in the subsequent error message.

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