net: fix small bug introduced by 48f7c4dd87fe

Fixes #6063

R=golang-dev, r, dave
CC=dvyukov, golang-dev
https://golang.org/cl/12586043

src/pkg/net/fd_windows.go

@@ -257,7 +257,7 @@ func (fd *netFD) init() error {
 	fd.wop.runtimeCtx = fd.pd.runtimeCtx
 	if !canCancelIO {
 		fd.rop.errc = make(chan error)
-		fd.rop.errc = make(chan error)
+		fd.wop.errc = make(chan error)
 	}
 	return nil
 }