[media] exynos4-is: Remove WARN_ON() from __fimc_pipeline_close()

It's not a critical error to call __fimc_pipeline_close() with missing
sensor subdev entity. Replace WARN_ON() with pr_warn() and return 0
instead of -EINVAL to fix control flow in some conditions.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>