Maintain the "hw_lro" configuration variable correctly.

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.

Differential Revision: https://reviews.freebsd.org/D4177
Submitted by: Shahar Klein <shahark@mellanox.com>
Sponsored by: Mellanox Technologies
MFC after: 3 days