mgw: Don't try to send packets while MDCX is underway

TC_two_crcx_and_one_mdcx_rtp_ho sometimes failed while trying to send an
RTP packet without a connected port. f_flow_modify re-bind()s the port,
sends the MDCX and then connect()s it after the MDCX ACK returned the
IP/port combination.
If the transmit timer fires off between the bind and connect the
resulting send call will fail.

Change-Id: Idf93ceb830a44dafa56430ab5178f05da6bdd6fb
1 file changed