video: fbdev: s3c-fb: Fix kernel-doc and set but not used warnings

Fix several W=1 warnings
- Updated kernel-doc as needed
- Deleted unused local variable, it was assigned but never used

v2:
  - Updated subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: linux-fbdev@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-25-sam@ravnborg.org