Make the AES availability cache actually cache things!

Ahem. I went to all the effort of setting up a wrapper function that
would store the result of the first call to aes_hw_available(), and
managed to forget to make it set the flag that said it _had_ stored
the result. So the underlying query function was being called every
time.

sshaes.c

@@ -134,8 +134,10 @@ static bool aes_hw_available_cached(void)
 {
     static bool initialised = false;
     static bool hw_available;
-    if (!initialised)
+    if (!initialised) {
         hw_available = aes_hw_available();
+        initialised = true;
+    }
     return hw_available;
 }