cirrusfb: do not allow unsupported pixel depth

Do not allow modes with unsupported pixel depth.  Otherwise, one can hang
a computer by setting incorrect value with fbset command.
Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/cirrusfb.c

@@ -567,9 +567,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
 	default:
 		dev_dbg(info->device,
 			"Unsupported bpp size: %d\n", var->bits_per_pixel);
-		assert(false);
-		/* should never occur */
-		break;
+		return -EINVAL;
 	}
 
 	if (var->xres_virtual < var->xres)