MFC r341784

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.
Differential Revision: https://reviews.freebsd.org/D18389