[BRIDGE]: fix error handling for add interface to bridge

Refactor how the bridge code interacts with kobject system.
It should still use kobjects even if not using sysfs.
Fix the error unwind handling in br_add_if.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>