net/af_xdp: fix max Rx packet length

net/af_xdp: fix max Rx packet length

Commit 1bb4a528c41f ("ethdev: fix max Rx packet length") clarified the
expected usage of the max_rx_pktlen and max_mtu values and implemented
some extra checks on these values to ensure they are sane. After this,
the AF_XDP PMD fails to initialise. The value for max_rx_pktlen which
represents the max size of the Ethernet frame was set to ETH_FRAME_LEN
(1514) and the max_mtu which represents the size of the payload was set
to the max size of the Ethernet frame. This did not make sense, as
naturally the maximum frame size should be greater than the payload
size.

Fix this by setting the max_rx_pktlen equal to the max size of the
Ethernet frame as expected, and the max MTU equal to the max_rx_pktlen
less the overhead which is set to the size of an Ethernet header plus
CRC.

Fixes: 1bb4a528c41f ("ethdev: fix max Rx packet length")

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>