sfxge: fix prefast warning in falconsiena_tx_qcreate

Keep prefast happy by returning the initial queue index
from falconsiena_tx_qcreate(). No change in behaviour, as
etxo_qcreate already zeros *addedp before the call.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days