uan: fix address problem in uan-mac-cw

I was trying to execute the uan-cw-example but, while the execution had no errors, the average values displayed were always zero (meaning that the sink node was not receiving any packets). Then, I discovered a problem in the UanMacCw class that prevented packets from being forwarded up. This problem was due to the use of an uninitialized attribute (m_address). An attribute with the same name also exists in the parent class UanMac, and the method SetAddress is used to set its value (while no method exists for the m_address in the UanMacCw class). So, I removed m_address in the UanMacCw and replaced its use by calling the method GetAddress. Now the uan-cw-example works properly.

This merge request has the fix I just described.