sfxge(4): fix ignoring function return value

fix PreFAST issue, add missing annotation that function return value
should not be ignored. Fix alignment.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18096