video: fbdev: hgafb: Fix kernel-doc warnings

Fix kernel-doc comments.

v2:
  - Updated subject (Lee)

v3:
  - Add space after ':' (Randy)
  - Fix name of function in comment

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ferenc Bakonyi <fero@drama.obuda.kando.hu>
Cc: linux-nvidia@lists.surfsouth.com
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-2-sam@ravnborg.org