Master loopback mode has a built-in streaming data generator and checker logic. In this
mode, the core generates streaming traffic on M_AXIS_MASTER. The same
traffic (after processing by target IP) when fed to S_AXIS_MASTER
interface compares the transaction generated at M_AXIS_MASTER versus a
transaction received at S_AXIS_MASTER. Fields TDATA, TSTRB, and TKEEP
are considered for comparison, and error count is reported through
axis_err_count port.