sim: Match structure definition better

This padding field is defined as a u8 and a u16.  Clarify the marshaling
code to match instead of just looping over bytes.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/tlv.rs b/sim/src/tlv.rs
index 39bb1b6..32e006f 100644
--- a/sim/src/tlv.rs
+++ b/sim/src/tlv.rs
@@ -291,9 +291,8 @@
 
                 // The dependency.
                 protected_tlv.push(dep.id);
-                for _ in 0 .. 3 {
-                    protected_tlv.push(0);
-                }
+                protected_tlv.push(0);
+                protected_tlv.write_u16::<LittleEndian>(0).unwrap();
                 protected_tlv.push(dep.version.major);
                 protected_tlv.push(dep.version.minor);
                 protected_tlv.write_u16::<LittleEndian>(dep.version.revision).unwrap();