video: sunxi: Change sunxi_get_mon_desc() to not return NULL for the default case

When building with gcc 9.2.0, the following build warning was seen:

  drivers/video/sunxi/sunxi_display.c: In function 'video_hw_init':
  drivers/video/sunxi/sunxi_display.c:1217:2:
    error: '%s' directive argument is null [-Werror=format-overflow=]

Change sunxi_get_mon_desc() to not return NULL for the default case,
to fix the compiler warning.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/video/sunxi/sunxi_display.c b/drivers/video/sunxi/sunxi_display.c
index 31f0aa7..4e1720e 100644
--- a/drivers/video/sunxi/sunxi_display.c
+++ b/drivers/video/sunxi/sunxi_display.c
@@ -1014,7 +1014,6 @@
 static const char *sunxi_get_mon_desc(enum sunxi_monitor monitor)
 {
 	switch (monitor) {
-	case sunxi_monitor_none:		return "none";
 	case sunxi_monitor_dvi:			return "dvi";
 	case sunxi_monitor_hdmi:		return "hdmi";
 	case sunxi_monitor_lcd:			return "lcd";
@@ -1023,8 +1022,9 @@
 	case sunxi_monitor_composite_ntsc:	return "composite-ntsc";
 	case sunxi_monitor_composite_pal_m:	return "composite-pal-m";
 	case sunxi_monitor_composite_pal_nc:	return "composite-pal-nc";
+	case sunxi_monitor_none:		/* fall through */
+	default:				return "none";
 	}
-	return NULL; /* never reached */
 }
 
 ulong board_get_usable_ram_top(ulong total_size)