spl: mmc: fix switch statement

If CONFIG_SPL_LIBCOMMON_SUPPORT is not defined there is a lone case statement
at the end of the switch leading to a compile error.
Remove the offending case statement.

| common/spl/spl_mmc.c:339:7: error: label at end of compound statement

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Marek Vasut <marex@denx.de>

common/spl/spl_mmc.c

@@ -342,7 +342,6 @@ static int spl_mmc_load_image(struct spl_image_info *spl_image,
 			return err;
 
 		break;
-	case MMCSD_MODE_UNDEFINED:
 #ifdef CONFIG_SPL_LIBCOMMON_SUPPORT
 	default:
 		puts("spl: mmc: wrong boot mode\n");