mlx5en: maintain hw_lro conf var correctly

[markb_mellanox.com]

Summary

Setting sysctl dev....conf.hw_lro may fail if the net device lro is
turned off. Due to the nature of our sysctl handler we need to set the
values back to 0 and issue an error

[• hselasky]

https://svnweb.freebsd.org/changeset/base/291068