ftgmac100: compute maximum frame size depending on the protocol

ftgmac100: compute maximum frame size depending on the protocol

The maximum frame size includes the CRC and depends if a VLAN tag is
inserted or not. Adjust the frame size limit in the transmit handler
using on the FTGMAC100State buffer size and in the receive handler use
the packet protocol.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180530061711.23673-2-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>