UniCore32-bugfix: fix mismatch return value of __xchg_bad_pointer

When disintegrate system.h, I left an error in asm/cmpxchg.h, which
will result in following error:

arch/unicore32/include/asm/cmpxchg.h: In function '__xchg':
arch/unicore32/include/asm/cmpxchg.h:38: error: void value not ignored as it ought to be
Signed-off-by: NGuan Xuetao <gxt@mprc.pku.edu.cn>

arch/unicore32/include/asm/cmpxchg.h

@@ -35,7 +35,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 			: "memory", "cc");
 		break;
 	default:
-		ret = __xchg_bad_pointer();
+		__xchg_bad_pointer();
 	}
 
 	return ret;