cxgbe/t4_tom: Detach the toep from the tcpcb when entering TIME_WAIT.

The kernel used to call tod_pcb_detach when entering TIME_WAIT but that
seems to have changed, likely with the TIME_WAIT overhaul in the kernel
some time ago. Catch up by having the driver perform the detach.

The hardware does not handle TIME_WAIT so it's important to detach and
let the kernel arm the 2MSL timer to deal with it.

Reported by: Sony Arpita Das @ Chelsio
Reviewed by: jhb
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45990