sfxge(4): fix default RSS context check on Siena

Default RSS context check is carried out during filter
insertion on Siena and it needs to be fixed

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18076