cxgbe/t4_tom: restore socket's protosw before entering TIME_WAIT.

This fixes a panic due to stale so->so_proto if t4_tom is unloaded and
one or more connections that were previously offloaded are still around
in TIME_WAIT state.

Reviewed by: jhb@
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29503