hw/net/stellaris_enet: Rewrite tx fifo handling code

hw/net/stellaris_enet: Rewrite tx fifo handling code

The datasheet is clear that the frame length written to the DATA
register is actually stored in the TX FIFO; this means we don't
need to keep both tx_frame_len and tx_fifo_len state separately.

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