hw/net/stellaris_enet: Correct handling of packet padding

hw/net/stellaris_enet: Correct handling of packet padding

The PADEN bit in the transmit control register enables padding of short
data packets out to the required minimum length. However a typo here
meant we were adjusting tx_fifo_len rather than tx_frame_len, so the
padding didn't actually happen. Fix this bug.

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