opencores_eth: flush queue whenever can_receive can go from false to true

The following registers control whether MAC can receive frames:
- MODER.RXEN bit that enables/disables receiver;
- TX_BD_NUM register that specifies number of RX descriptors.
Notify QEMU networking core when the MAC is ready to receive frames.
Discard frame and raise BUSY interrupt when the frame arrives but the
current RX descriptor is not empty.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>