ssh: use a more straightforward return value

Change-Id: Ie5ee95efe4924f75719087c6fe8d4867607934bf
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/653198
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: cuishuang

Diff for: ssh/tcpip.go

@@ -459,7 +459,7 @@ func (c *Client) dial(laddr string, lport int, raddr string, rport int) (Channel
 		return nil, err
 	}
 	go DiscardRequests(in)
-	return ch, err
+	return ch, nil
 }
 
 type tcpChan struct {