ethdev: allow maximum packet length to less than 1518

ethdev: allow maximum packet length to less than 1518

In ethdev, it ignores setting maximum packet length to less than
1518. The changes is to remove this limitation and let less than
1518 can be set for 'maximum packet length'.

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Signed-off-by: Jing Chen <jing.d.chen@intel.com>
Acked-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: Heqing Zhu <heqing.zhu@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>