sfxge(4): report support for Tx checksum op descriptors

FreeBSD driver needs a patch to provide a means for packets
which do not need checksum offload but have flow ID set
to avoid hitting only the first Tx queue (which has been used
for packets not needing checksum offload).

This should be possible on Huntington, Medford or Medford2 chips
since these support toggling checksum offload on any given queue
dynamically by means of pushing option descriptors.

The patch for FreeBSD driver will then need a means to figure out
whether the feature can be used, and testing adapter family might
not be a good solution.

This patch adds a feature bit specifically to indicate support
for checksum option descriptors. The new feature bits may have
more users in future, apart from the mentioned FreeBSD patch.

Submitted by: Ivan Malov <Ivan.Malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18388