submodule(CoupledL2): set TXREQ with the highest priority (OpenXiangS…

…han#3600)

The QoS *Priority Value* for packets is dependent on the source type and
the class of traffic with ascending values of QoS indicating a higher
priority level. In the previous design QoS is always set to zero,
resulting in a rather low use ratio of CMN.

This commit sets all the TXREQ requests with the highest priority to
fully utilize the resources in HN-Fs.