media: rkisp1: Get rid of unused variable warning

If CONFIG_OF is not selected, the compiler will complain:

drivers/staging/media/rkisp1/rkisp1-dev.c: In function ‘rkisp1_probe’:
drivers/staging/media/rkisp1/rkisp1-dev.c:457:22: warning: unused variable ‘node’ [-Wunused-variable]
  457 |  struct device_node *node = pdev->dev.of_node;

Rework the code slightly and make the compiler happy.

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>