The failing loopback case is a negative test: its TX buffer deliberately has
no FCS, and the test expects a CRC error. The DTCR=1 case that does carry an
FCS is delivered intact and passes, so the DMA is not dropping four bytes.
The sole remaining mismatch is one bit: the test wants RMD1=0x6B00
(ERR|FRAM|CRC|STP|ENP) and MAME's am79c90 reports 0x4B00 -- it never sets
RMD1_FRAM. No MAME-tree change made; reported for a second reader.