The functions smsc9220_receive_by_chunks and smsc9220_send_by_chunks are
supposed to implement receiving and sending packets by chunks. However,
the functions SMSC9220_EMAC::low_level_input and SMSC9220_EMAC::link_out,
which call them respectively, already require or assemble the full packet.
Also, smsc9220_receive_by_chunks doesn't implement the "chunks" part.

This commit renames the functions to smsc9220_receive_packet and
smsc9220_send_packet. The functions now do their operations by word
instead of by bytes. The functions SMSC9220_EMAC::low_level_input and
SMSC9220_EMAC::link_out already handle allocation, continuity and word
alignment of the packet buffer.