fix error output of brctl if bridging is not supported.

fix error output of brctl if bridging is not supported.

The following error is displayed during network configuration on any device that has DEVICE_TYPE set to something else then router, or has brctl functionality disabled through some other method:

"brctl: SIOCGIFBR: Package not installed"

This is due to the unbridge function being called, which uses "brctl show" to determine if the interface is bridged.

Signed-off-by: Ithamar R. Adema <ithamar.adema@team-embedded.nl>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@20218 3c298f89-4303-0410-b956-a3cf2f4a3e73