fbdev: hgafb: fix kernel-doc comments

Fix kernel-doc warnings found when using "W=1".

hgafb.c:370: warning: No description found for return value of 'hgafb_open'
hgafb.c:384: warning: No description found for return value of 'hgafb_release'
hgafb.c:406: warning: No description found for return value of 'hgafb_setcolreg'
hgafb.c:425: warning: No description found for return value of 'hgafb_pan_display'
hgafb.c:425: warning: expecting prototype for hga_pan_display(). Prototype was for hgafb_pan_display() instead
hgafb.c:455: warning: No description found for return value of 'hgafb_blank'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ferenc Bakonyi <fero@drama.obuda.kando.hu>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-nvidia@lists.surfsouth.com
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Helge Deller <deller@gmx.de>