sfxge(4): fix potential buffer overflow in Tx queue init

Improve error checking to avoid a caller overflowing the MCDI
request buffer if the requested TXQ size was excessively large.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18067