sfxge(4): update unicast filter insertion algorithm

As unicast filters are not chained, we should always try to insert the
specific filter for our MAC address, and then try to insert the unicast
mis-match filter if that fails or all unicast has been requested.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6295