bridge: Ignore "null" interfaces as required.

bridge: Ignore "null" interfaces as required.

Commit bae7208e91a0 (bridge: Refactor bridge_reconfigure().)
introduced a regression in which the switch would attempt to
instantiate "null" interfaces in the datapath. This would, of
course, fail and trigger a warning.  Though harmless, these
warnings confused users.

Signed-off-by: Ethan Jackson <ethan@nicira.com>