bridge: Enable support for access and native VLAN ports on bonds.

Since Open vSwitch's inception we've disabled the use of bonds as access
ports, for no particularly good reason.  This also unintentionally
prevented bonds from being used as native VLAN ports.

This commit removes the code that prevented using bonds these ways

Reported-and-tested-by: "Michael A. Collins" <mike.a.collins@ark-net.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>