sim: Remove useless `return`s

Apply clippy suggestions to directly result in values instead of
returning those values at the end of a function.  Better matches common
Rust style.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/simflash/src/lib.rs b/sim/simflash/src/lib.rs
index dad0bca..009148c 100644
--- a/sim/simflash/src/lib.rs
+++ b/sim/simflash/src/lib.rs
@@ -138,7 +138,7 @@
             }
             offset -= size;
         }
-        return None;
+        None
     }
 
 }