net/e1000: fix jumbo frame flag condition for MTU set

net/e1000: fix jumbo frame flag condition for MTU set

The jumbo frame uses the 'RTE_ETHER_MAX_LEN' as boundary condition,
but 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' and
overhead, that perhaps impacts the cases of the jumbo frame related.

Fixes: ef990fb56e55 ("net/e1000: convert to new Rx offloads API")
Cc: stable@dpdk.org
Signed-off-by: Steve Yang <stevex.yang@intel.com>
Acked-by: Jeff Guo <jia.guo@intel.com>