sfxge(4): use n Tx queues instead of n + 2 on EF10 HW

On EF10 HW we can avoid sending packets without checksum offload
or with IP-only checksum offload to dedicated queues. Instead, we
can use option descriptors to change offload policy on any queue
during runtime. Thus, we don't need to create two dedicated queues.

Submitted by: Ivan Malov <Ivan.Malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18390