[low-power] keep configured CSL channel for child (#5432)

A SSED uses the Child Update Request to start CSL synchronization. If
the CSL Channel TLV is not present in the message, the parent assigns
the Thread Network channel as the CSL channel for this child during
the attachment.

However, it may happen that a child changes its CSL channel to a
different one with a Child Update Request and decides to keep using
it. In a subsequent Child Update Request it could omit the CSL Channel
TLV (no changes) but the parent would change its CSL channel to the
Thread Network channel. This commit avoids that.

src/core/thread/mle_router.cpp

@@ -2540,7 +2540,7 @@ void MleRouter::HandleChildUpdateRequest(const Message &         aMessage,
         {
             child->SetCslChannel(static_cast<uint8_t>(cslChannel.GetChannel()));
         }
-        else
+        else if (child->GetCslChannel() == 0)
         {
             child->SetCslChannel(Get<Mac::Mac>().GetPanChannel());
         }