Don't look for ethernet attached to axi_dma when MM2S channel disabled.

This script didn't work for a design in which I had an axi_dma peripheral installed.  I configured that peripheral without an MM2S interface, which caused the script to crash when I ran it.

This fix skips past the "look for ethernet IP attached to the MM2S bus" stuff when C_INCLUDE_MM2S is zero.

--wpd