sh: Tidy up dependencies for SH-2 build.

SH-2 can presently get in to some pretty bogus states, so
we tidy up the dependencies a bit and get it all building
again.

This gets us a bit closer to a functional allyesconfig
and allmodconfig, though there are still a few things to
fix up.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/include/asm-sh/system.h b/include/asm-sh/system.h
index fb22fc3..7c75045 100644
--- a/include/asm-sh/system.h
+++ b/include/asm-sh/system.h
@@ -126,7 +126,7 @@
 #define smp_read_barrier_depends()	do { } while(0)
 #endif
 
-#define set_mb(var, value) do { xchg(&var, value); } while (0)
+#define set_mb(var, value) do { (void)xchg(&var, value); } while (0)
 
 /*
  * Jump to P2 area.