media: staging: media: ipu3: add missing kernel-doc 'struct' keywords

Two structs where missing the kernel-doc 'struct' keyword
in their kernel-doc documentation.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>