libifconfig: Skip bridge VLAN config for span members

bridge(4) doesn't support BRDGGIFVLANSET for span members, which
means if a span interface is configured, libifconfig will fail to
fetch bridge members. Skip this for IFBIF_SPAN members.

PR: 292634
MFC after: 3 days
Reported by: Emrion <kmachine@free.fr>
Reviewed by: pouria, zlei
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56694