ARM: hisilicon: hikey: do not rely on random stack value

If CONFIG_MMC_DW is not defined the return value of
init_dwmmc should not rely on a random stack value.

Instead indicate that no error occured.

The problem was indicated by cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

board/hisilicon/hikey/hikey.c

@@ -345,7 +345,7 @@ int board_init(void)
 
 static int init_dwmmc(void)
 {
-	int ret;
+	int ret = 0;
 
 #ifdef CONFIG_MMC_DW