nb/intel/x4x/raminit: Initialise async variable

It could end up not initialized which causes it not to build with
clang.

Change-Id: I3be9477d836123aaa87c9bebb41c1ec34689a771
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/19736
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Martin Roth <martinroth@google.com>

src/northbridge/intel/x4x/raminit_ddr2.c

@@ -714,7 +714,7 @@ static void timings_ddr2(struct sysinfo *s)
 
 static void dll_ddr2(struct sysinfo *s)
 {
-	u8 i, j, r, reg8, clk, async;
+	u8 i, j, r, reg8, clk, async = 0;
 	u16 reg16 = 0;
 	u32 reg32 = 0;
 	u8 lane;