mips: ath79: ar933x: Avoid warning with gcc5

GCC 5.3 report a warning: 'upper' and 'lower' may be used
uninitialized in this function [-Wmaybe-uninitialized].
Compiler might need explicit initializer.
Signed-off-by: NWills Wang <wills.wang@live.com>

arch/mips/mach-ath79/ar933x/ddr.c

@@ -268,6 +268,8 @@ void ddr_tap_tuning(void)
 	dir = 1;
 	tap = readl(regs + AR71XX_DDR_REG_TAP_CTRL0);
 	val = tap;
+	upper = tap;
+	lower = tap;
 	while (!done) {
 		err = 0;