hw/net/stellaris_enet: Get rid of rx_fifo pointer

hw/net/stellaris_enet: Get rid of rx_fifo pointer

The rx_fifo pointer is awkward to migrate, and is actually
redundant since it is always possible to determine it from
the current rx[].len/.data and rx_fifo_len. Remove both
rx_fifo and rx_fifo_len from the state, replacing them with
a simple rx_fifo_offset which points at the current location
in the RX fifo.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>