sh64: Trivial build fixes.

While we've been sorting out the toolchain fiasco, some of
the code has suffered a bit of bitrot. Building with GCC4
also brings up some more build warnings. Trivial fixes for
both issues.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/include/asm-sh64/system.h b/include/asm-sh64/system.h
index 87ef6f1..b1598c2 100644
--- a/include/asm-sh64/system.h
+++ b/include/asm-sh64/system.h
@@ -64,7 +64,7 @@
 #define smp_read_barrier_depends()	do { } while (0)
 #endif /* CONFIG_SMP */
 
-#define set_rmb(var, value) do { xchg(&var, value); } while (0)
+#define set_rmb(var, value) do { (void)xchg(&var, value); } while (0)
 #define set_mb(var, value) set_rmb(var, value)
 
 /* Interrupt Control */