arch/tile: Fix atomic_read() definition to use ACCESS_ONCE

This adds the volatile cast which forces the compiler to emit the load.
Suggested by Peter Zijlstra <peterz@infradead.org>.
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/include/asm/atomic.h

@@ -32,7 +32,7 @@
  */
 static inline int atomic_read(const atomic_t *v)
 {
-       return v->counter;
+	return ACCESS_ONCE(v->counter);
 }
 
 /**