sim: Combine a split conditional

Instead of nesting ifs, just use the conditional with an and operator.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/image.rs b/sim/src/image.rs
index 37c59ad..6095ba4 100644
--- a/sim/src/image.rs
+++ b/sim/src/image.rs
@@ -512,12 +512,10 @@
                 fails += 1;
             }
 
-            if self.is_swap_upgrade() {
-                if !self.verify_images(&flash, 1, 0) {
-                    warn!("Secondary slot FAIL at step {} of {}",
-                          i, total_flash_ops);
-                    fails += 1;
-                }
+            if self.is_swap_upgrade() && !self.verify_images(&flash, 1, 0) {
+                warn!("Secondary slot FAIL at step {} of {}",
+                    i, total_flash_ops);
+                fails += 1;
             }
         }