net/nfp: fix jumbo frame flag condition for MTU set

net/nfp: fix jumbo frame flag condition for MTU set

The jumbo frame uses the 'RTE_ETHER_MAX_LEN' as boundary condition.
If the Ether overhead is larger than 18 when it supports VLAN tag,
that will cause the jumbo flag rx offload is wrong when MTU size is
'RTE_ETHER_MTU'.

This fix will change the boundary condition with 'RTE_ETHER_MTU',
that perhaps impacts the cases of the jumbo frame related.

Fixes: d4a27a3b092a ("nfp: add basic features")
Cc: stable@dpdk.org
Signed-off-by: Steve Yang <stevex.yang@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>