RMII receive doesn't receive packets with incorrect preamble

[shuchitak]

Tests test_4_2_4.py and test_4_1_9.py fail since the DUT is expected to receive frames with preambles that are either of incorrect length or have incorrect nibble values.

The MII receiver is able to handle incorrect preambles and successfully receive the packet while the RMII receiver drops them.

As part of this issue

• Investigate if supporting invalid preamble is a requirement. If yes,
• Add this functionality in the 4b and 1b receiver code, else
• Document this limitation in the RMII documentation