The standard MAC layer doesn't detect invalid SFDs (4_1_7)

[pthedinger]

The SFD (Start of Frame Delimiter) is defined as being 10101011 which comes after a preamble which is a sequence of 10101010s. However, in the hardware we only look for the 0xd nibble to indicate the SFD.

The issue is that UNH/IOL test 4_1_7 expects a frame with the SFD value of 10011011 to be thrown away. The standard MAC (mii_ethernet_mac) does not detect this error and therefore incorrectly accepts this packet.