hw/net/dp8393x: fix integer underflow in dp8393x_do_transmit_packets()

hw/net/dp8393x: fix integer underflow in dp8393x_do_transmit_packets()

An integer underflow could occur during packet transmission due to 'tx_len' not
being updated if SONIC_TFC register is set to zero. Check for negative 'tx_len'
when removing existing FCS.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1899722
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-id: 20201124092445.658647-1-mcascell@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>