Check TF_TOE not the tod pointer to determine if TOE is active.

The TF_TOE flag is the check used in the rest of the network stack to
determine if TOE is active on a socket. There is at least one path in
the cxgbe(4) TOE driver that can leave the tod pointer non-NULL on a
socket not using TOE.

Reported by: Sony Arpita Das <sonyarpitad@chelsio.com>
Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D26803