net/e1000: fix max Rx packet size

net/e1000: fix max Rx packet size

According to E1000_ETH_OVERHEAD definition, max_rx_pkt_len contains
one VLAN tag size. Therefore when config RLPML register, if dual VLAN
not enabled there is no need to add VLAN tag size to max_rx_pkt_len,
otherwise only one another VLAN tag size should be added to.

Fixes: e51abef39382 ("igb: fix max RX packet size and support dual VLAN")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Tested-by: Lingli Chen <linglix.chen@intel.com>
Acked-by: Haiyue Wang <haiyue.wang@intel.com>