[media] exynos4-is: Fix potential null pointer dereference in mipi-csis.c

When 'node' is NULL, the print statement tries to dereference it.
Hence replace the variable with the one that is accessible.

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>