[mle] fix incorrect error code for malformed CSL Timeout TLV (#13610)

The `default:` arm for `Tlv::Find<CslTimeoutTlv>()` set `error` to
`kErrorNone` instead of `kErrorParse` when the TLV was malformed,
unlike every other TLV parsed in this function. Because
`LogProcessError()` only logs a warning when the error is non-zero, a
malformed CSL Timeout TLV (e.g. a truncated value) was silently
dropped with no diagnostic, instead of being logged as a parse error
like other malformed TLVs in a Child Update Request.

src/core/thread/mle_ftd.cpp

@@ -2355,7 +2355,7 @@ void Mle::HandleChildUpdateRequestOnParent(RxInfo &aRxInfo)
         case kErrorNotFound:
             break;
         default:
-            ExitNow(error = kErrorNone);
+            ExitNow(error = kErrorParse);
         }
 
         switch (Tlv::Find<CslChannelTlv>(aRxInfo.mMessage, cslChannelTlvValue))