openrisc: xchg: fix `computed is not used` warning

When building allmodconfig this warning shows.

  fs/ocfs2/file.c: In function 'ocfs2_file_write_iter':
  ./arch/openrisc/include/asm/cmpxchg.h:81:3: warning: value computed is
  not used [-Wunused-value]
    ((typeof(*(ptr)))__xchg((unsigned long)(with), (ptr), sizeof(*(ptr))))
     ^

Applying the same patch logic that was done to the cmpxchg macro.
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/include/asm/cmpxchg.h

@@ -77,7 +77,11 @@ static inline unsigned long __xchg(unsigned long val, volatile void *ptr,
 	return val;
 }
 
-#define xchg(ptr, with) \
-	((typeof(*(ptr)))__xchg((unsigned long)(with), (ptr), sizeof(*(ptr))))
+#define xchg(ptr, with) 						\
+	({								\
+		(__typeof__(*(ptr))) __xchg((unsigned long)(with),	\
+					    (ptr),			\
+					    sizeof(*(ptr)));		\
+	})
 
 #endif /* __ASM_OPENRISC_CMPXCHG_H */