LAN9220: Correct initialisation and reception

* Ensure certain flags are inspected before performing operations as recommended in the datasheet.
* Clear RX FIFO after reception of data to ensure 4-byte end alignment so subsequent data can be received.
* Remove unused structure member in `smsc9220_eth_dev_data_t` data type.
* Rename certain functions by using common abbreviations.

This fix ensures that MPS3 FPGA passes all MDH EMAC tests.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>