tile: fix comment bug in sys_cmpxchg description

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/kernel/intvec_32.S

@@ -1609,7 +1609,7 @@ ENTRY(sys_cmpxchg)
 	  * Because of C pointer arithmetic, we want to compute this:
 	  *
 	  * ((char*)atomic_locks +
-	  *  (((r0 >> 3) & (1 << (ATOMIC_HASH_SIZE - 1))) << 2))
+	  *  (((r0 >> 3) & ((1 << ATOMIC_HASH_SHIFT) - 1)) << 2))
 	  *
 	  * Instead of two shifts we just ">> 1", and use 'mm'
 	  * to ignore the low and high bits we don't want.