cfb2: Fix cfb2_setpixel() return value

On cfb2, pixel values are in the range 0..3, not 0..15.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
diff --git a/drawops/cfb2.c b/drawops/cfb2.c
index e3c92f7..04af886 100644
--- a/drawops/cfb2.c
+++ b/drawops/cfb2.c
@@ -39,7 +39,7 @@
 
 static pixel_t cfb2_getpixel(u32 x, u32 y)
 {
-    return (screen[y*screen_width+x/4] >> (2*(3- (x & 3)))) & 15;
+    return (screen[y*screen_width+x/4] >> (2*(3- (x & 3)))) & 3;
 }
 
 const struct drawops cfb2_drawops = {