xchg is atomic with side-effects

clang doesn't know about the side effect, so we have to tell it
that it's okay not to care about the result.

Change-Id: Ib11890bff6779e36cf09c178d224695ea16a8ae8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/783
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/cpu/x86/lapic.h

@@ -108,7 +108,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 
 static inline void lapic_write_atomic(unsigned long reg, unsigned long v)
 {
-	xchg((volatile unsigned long *)(LAPIC_DEFAULT_BASE+reg), v);
+	(void)xchg((volatile unsigned long *)(LAPIC_DEFAULT_BASE+reg), v);
 }