media: exynos-gsc/exynos4-is: add missing 'struct' to kerneldoc

Fix kernel-doc warnings due to missing 'struct' keyword.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>