sfxge(4): don't use Tx descriptor push with TSO option descriptors

It is not safe to push TSO option descriptors if pacer bypass is
enabled, so to make sure that doesn't happen never push TSO option
descriptors.

Submitted by: Mark Spender <mspender at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8952