Use tcp_state_change() in the cxgbe(4) TOE module.

r254889 added tcp_state_change() as a centralized place to log state
changes in TCP connections for DTrace. r294869 and r296881 took
advantage of this central location to manage per-state counters.
However, TOE sockets were still performing some (but not all) state
change updates via direct assignments to t_state. This resulted in
state counters underflowing when TOE was in use. Fix by using
tcp_state_change() when changing a TOE connection's state.

Reviewed by: np, markj
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D17915