sfxge(4): prepare the number of Tx queues on event queue 0 to become variable

The number of Tx queues on event queue 0 can depend on the NIC family type,
and this property will be leveraged by future patches.
This patch prepares the code for this change.

Submitted by: Ivan Malov <Ivan.Malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18389