bridge: Fix spurious warnings about capabilities

Mask off the bits we don't care about when checking that capabilities
of the member interfaces have been disabled as intended.

Submitted by: Ryan Moeller <ryan@ixsystems.com>
Reviewed by: kristof, mav
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D18924