fbdev: sh_mobile_lcdcfb: Fix ARGB32 overlay format typo

When configurating a CHn Source Image Format Register (LDBBSIFR), one
should use the corresponding LDBBSIFR_RPKF_* definition for overlay
planes, not the DDFR_PKF_* definition for the primary plane.

Fortunately both definitions resolve to the same value, so this bug did
not cause any harm.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Helge Deller <deller@gmx.de>