fbdev: ipu-v3: clean up kernel-doc warnings

fbdev: ipu-v3: clean up kernel-doc warnings

Correct all kernel-doc warnings:
- fix a typedef kernel-doc comment
- mark a list_head as private
- use Returns: for function return values

Warning: include/video/imx-ipu-image-convert.h:31 struct member 'list' not
 described in 'ipu_image_convert_run'
Warning: include/video/imx-ipu-image-convert.h:40 function parameter
 'ipu_image_convert_cb_t' not described in 'void'
Warning: include/video/imx-ipu-image-convert.h:40 expecting prototype for
 ipu_image_convert_cb_t(). Prototype was for void() instead
Warning: include/video/imx-ipu-image-convert.h:66 No description found for
 return value of 'ipu_image_convert_verify'
Warning: include/video/imx-ipu-image-convert.h:90 No description found for
 return value of 'ipu_image_convert_prepare'
Warning: include/video/imx-ipu-image-convert.h:125 No description found for
 return value of 'ipu_image_convert_queue'
Warning: include/video/imx-ipu-image-convert.h:163 No description found for
 return value of 'ipu_image_convert'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>