luci-base: network.lua: fix bridge related functions
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 <5a0472fdb9489ec6419a1f506715354ce224b5a7@tano-systems.com>

hnyman/luci,hnyman/luci,openwrt/luci,tobiaswaldvogel/luci,rogerpueyo/luci,rogerpueyo/luci,rogerpueyo/luci,tobiaswaldvogel/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,openwrt/luci,openwrt/luci,tobiaswaldvogel/luci,rogerpueyo/luci,openwrt/luci,hnyman/luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,hnyman/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,openwrt/luci,tobiaswaldvogel/luci,lbthomsen/openwrt-luci,hnyman/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,rogerpueyo/luci,openwrt/luci,openwrt/luci,tobiaswaldvogel/luci,openwrt/luci,rogerpueyo/luci,hnyman/luci,rogerpueyo/luci,rogerpueyo/luci,lbthomsen/openwrt-luci