luci-base: network.lua: fix bridge related functions

This commit fixes these bugs:

1. Functions interface.bridge_id() and interface.bridge_stp() uses self.br field
   that is not exists at all

2. Function interface.is_bridgeport() return true only for bridge interface, not for
   bridge port interfaces.

Signed-off-by: Anton Kikin <a.kikin@tano-systems.com>