[packages] net/iperf: fix a bug in single thread mode

[packages] net/iperf: fix a bug in single thread mode

root@OpenWrt:/# iperf -c 192.168.20.100 -i 60
------------------------------------------------------------
Client connecting to 192.168.20.100, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 192.168.20.1 port 53214 connected with 192.168.20.100 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0--7988.8 sec  0.00 Bytes  -0.00 bits/sec
[ ID] Interval       Transfer     Bandwidth
[  3] -7988.8--7928.8 sec  0.00 Bytes  0.00 bits/sec

git-svn-id: svn://svn.openwrt.org/openwrt/packages@14776 3c298f89-4303-0410-b956-a3cf2f4a3e73