hw/net/stellaris_enet: Restructure tx_fifo code to avoid buffer overrun

The current tx_fifo code has a corner case where the guest can overrun
the fifo buffer: if automatic CRCs are disabled we allow the guest to write
the CRC word even if there isn't actually space for it in the FIFO.
The datasheet is unclear about exactly how the hardware deals with this
situation; the most plausible answer seems to be that the CRC word is
just lost.

Implement this fix by separating the "can we stuff another word in the
FIFO" logic from the "should we transmit the packet now" check. This
also moves us closer to the real hardware, which has a number of ways
it can be configured to trigger sending the packet, some of which we
don't implement.

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